Secretary of Defense Harold Brown arrived here yesterday for a three-day visit of reassurance for South Korea as the United States prepares to make the first formal withdrawal of its ground combat forces.

Brown's trip is timed for the arrival this week of a new squadron of U.S. fighter planes, part of the American plan to bolster South Korean air defenses as ground troops are pulled out.

He also will take part in a realignment of the military command that is designed to give South Koreans more influence and control in day-to-day planning and operations of their joint forces. That realignment, in the works for several years, was hastened by the impending troop reduction.

In an airport statement, Brown reiterated the usual U.S. reassurances about honoring America's security commitment to South Korea.

"We take this commitment with utmost seriousness," he said. The reduction of ground forces, he added, "will be taken in a prudent manner" in conjunction with an "overall strengthening" of the country's armed forces.

Under the Carter administration's delayed withdrawal plan, 3,600 troops are to leave the peninsuula this year. About two-thirds of them already left under a slow attrition policy of not replacing forces rotated home. The remainder will leave as a group sometime in mid-December.

Although still nervous about the departure of American forces, the South Korean government has now accepted as inevitable the Carter administration's plan to withdraw ground forces, U.S. officials here say.

However, a new element of concern - widely publicized by the government - was introduced last week with the discovery of a new tunnel that the North Koreans had allegedly dug underneath the demilitarized zone that separates the two countries.

The tunnel, about a mile long and extending into South Korea, was first suspected last summer but if was not officially confirmed until Oct. 17, when South Korean miners bored in a connecting tunnel.

American representatives at the military armistice commission meeting in Panmunjomdenounced the tunnel and both they and the South Korean government have presented it as an important bit of new evidence confirming North Korean plans for aggression.North Korea denied it built the tunnel.

Ever since the discovery, massive demonstrations have been held throughout South Korea in a display of national anger at the north. Such demonstrations do not occur without the government's approval, if not its encouragement.

There has been speculations that the South Korean government would offer the tunnel - the third of its kind to be found since 1974 - as proof that North Korean aggression continues and that the U.S. force reduction should be reconsidered. A government-controlled newspaper pointed editorially to the tunnel as a development that seems to require "revision" of the U.S. pullout plans.

By the end of this year, about 3,400 of the 32,000 ground forces will have been withdrown. That increment includes one infantry battalion of combat troops, support elements, and previously deactivated air defense and ground artillery missile battalions.

Another 2,600 troops are to be withdrawn next year including two more battalions from the 2d Infantry Division.

The American military leaders are vague on the schedule of reductions after 1979. The U.S. commander here, Gen. John W. Vessey Jr., told the local press last week that the post-1979 plans are "not clear". U.S. officials also stress that the United States will take other measures to strengthen South Korean defences as troops are withdrawn.

The principal evidence of that will be displayed on Wednesday when Brown flies to Taegu to watch the arrival of a new squadron of F4 jet fighters. The 12 new aircraft will bring to 62 the number of F4s stationed here.

Today Brown is to take part in the activation of a new combined forces command that will integrate South Korean commanders more intimately into the top joint command structure. Planned for several years, its introduction was hastened by the approach of the first formal U.S. withdrawal under the Carter administration plan.