A reduction in guerrilla attacks and shifting military tactics have cut in half the number of Salvadoran Army casualties during the past year, according to a Defense Ministry count.

The drop marks a sharp change from a year ago, when the official casualty count had doubled from the previous year following a long stretch of hard fighting with leftist rebels engaged in an offensive that had raised fears that the U.S-backed Army might collapse.

Defense Minister Carlos Eugenio Vides Casanova, in a report to the Legislative Assembly, said 3,108 soldiers were killed, wounded or reported missing in action during the year ended May 31. This compared with 6,815 for the same period the previous year.

Vides Casanova, who delivered the report to legislators in late July, said 1,055 officers and enlisted men were killed, 1,783 were wounded and 270 were listed as missing in action. In a similar report released a year earlier, the minister had counted totals of 2,292 killed, 4,195 wounded and 328 missing.

Military observers and U.S. officials here portrayed the drop as the fruit of more aggressive tactics by the 40,000-man Salvadoran armed forces. With more emphasis on mobile tactics designed to flush out the rebels, they said, there were fewer attacks in which guerrillas inflicted heavy casualties on stationary troops guarding installations or on exposed columns advancing toward villages taken by guerrilla forces whose comrades were waiting in ambush.

"That's where you really took a lot of casualties" during the 1982-83 reporting period, said a military observer. "And these are clearly on the decline."

One notable exception, these sources acknowledged, was a rebel attack in December on the garrison at El Paraiso in the northern province of Chalatenango. More than 100 government soldiers were killed by a 1,000-man rebel force attacking the poorly defended post.

But Salvadoran analysts noted that the number of casualties fell along with the general level of fighting, which they said was sharply below what it was during the previous reporting period. These analysts, one sympathetic to the government and the other to the leftist guerrilla cause, attributed the lull mostly to lack of initiative by the guerrillas in addition to new Army tactics.

Despite the reduction in combat, observers noted the ratio of dead to wounded remained at approximately one to two. This is an exceptionally high proportion, according to military experts, and has contributed to morale problems in the Salvadoran Army.

By comparison, U.S. forces in Vietnam counted one killed in every eight casualties, U.S. military officers said. A large factor in keeping down the number of U.S. deaths was swift helicopter evacuation of the wounded and attention from trained medics in the field and doctors in hospitals, they added.

With this in mind, the Reagan administration dispatched four UH1H Huey helicopters earmarked for medical evacuation in May. They were part of a $32 million emergency-aid package sent by executive order while Congress was debating an administration request for supplementary aid.

Since the reporting period cited by Vides Casanova ended May 31, however, these helicopters were believed to have little if any effect on the number of casualties reported by Vides Casanova.

President Reagan also sent 25 U.S. medics who have been training Salvadorans in battlefield and hospital care since July 1983, so far without denting the high ratio of dead to wounded. As a result, the key to over-all casualties as well as the number of dead still appeared to be the pace of fighting.

Since the first of the year, forces of the Farabundo Marti National Liberation Front, the overall guerrilla organization, have mounted only one major attack, briefly taking control of a dam at Cerron Grande in the center of the country before being driven off by helicopter-transported government reinforcements.

A member of the rebels' political leadership acknowledged in a recent interview that the guerrillas have lost "presence" on the ground in El Salvador and on the political stage abroad because of lack of military successes and President Jose Napoleon Duarte's promises of political reform in the government and armed forces.

The rebel leadership declared late last month, however, that persistent small-scale engagements have taken "more than a thousand Army casualties, including killed and wounded," since Duarte was inauguarated June 1.

"This obviously is not a picture that allows an affirmation that Duarte's Army holds the military initiative," the leadership said on the rebels' official Radio Venceremos.

A political analyst usually sympathetic to the guerrilla cause said, however, that guerrilla statistics appeared to be getting less reliable in recent months as the Army performance has improved.

Since statistics on guerrilla casualties reported by the Salvadoran Army also are considered by U.S. and Salvadoran analysts to be inflated, there is no way to gain an accurate picture of rebel casualties. But forced recruiting by guerrilla units recently has generated speculation that their losses may be high.

The rebel official, interviewed outside El Salvador, pointed to increased bombing by A37 Dragonfly jets and swift attacks by Salvadoran troops, particularly those transported by helicopter, as the principal problems faced by guerrilla units. U.S. officials, apparently with this in mind, are planning to send the Salvadoran Army about 10 more UH1H Huey helicopters for troop transport, part of a $70 million supplementary aid package approved by Congress last month