West Germany's new conservative government, which took office this month promising to keep national defenses strong, today unveiled military spending plans for next year that will barely keep ahead of projected inflation.

Finance Minister Gerhard Stoltenberg, detailing the 1983 budget approved in the Cabinet yesterday, told reporters defense spending would rise by 4.8 percent to $18.5 billion (46.18 billion deutsche marks). The government's forecasts point to an inflation rate next year of 4 percent, eroding most of the real effect of the rise.

The new budget represents a gain of only $40 million over the sum that former chancellor Helmut Schmidt had initially earmarked for defense next year. Although the military significance of this increase was uncertain -- the extra funds received were for "construction projects" -- the fact that any rise was approved carried at least symbolic importance.

"It's surely a political sign," said Bonn Defense Ministry spokesman Norbert Huebner.

Compared with many areas of federal spending, which face substantial cuts or caps as part of Chancellor Helmut Kohl's austerity drive, defense was favored. Proposed reductions in welfare benefits, delays in pension increases and higher national insurance contributions have already brought mounting criticism from powerful trade unions. Workers would not have taken kindly to a disproportionate jump in military spending.

Overall, the 1983 budget provides for a 2.9 percent rise in federal spending -- less than the slated increase for defense -- to $101.5 billion. Net public borrowing is planned to balloon again to $16.6 billion, as a result of growing demands for the broad unemployment benefits and in view of recession-depressed tax revenues.

Nevertheless, the prospect of little real growth in West German military spending next year is apt to disappoint the Reagan administration, which continues to call for greater defense efforts by the European allies. Defense Minister Manfred Woerner, is scheduled to arrive in Washington Nov. 7 for consultations preceding a visit by Kohl Nov. 15.

While in opposition, the conservatives regularly criticized Schmidt's center-left coalition for spending too little on defense. Last July, Friedrich Zimmermann, now interior minister, accused the government of failing to guarantee West Germany's security in the face of a Soviet military buildup.

He pointed out that the military's share of the federal budget had fallen from 27.1 percent in 1969, when the Social Democrats assumed power, to 18.2 percent in 1981. The new defense budget will again account for only 18.2 percent of total federal spending, in line with Schmidt's initial plans.

The announced budget also appears to fall far short of Bonn's pledge to the North Atlantic Treaty Organization to raise defense spending by 3 percent annually after inflation.

But Huebner, the Defense Ministry spokesman, cautioned that West Germany will show more of a real increase by NATO standards than might appear at first. This, he said, is because in calculating West Germany's contributions, NATO also includes such items outside the normal military budget as Bonn's military aid to Turkey, Portugal and Greece as well as its supporting costs for the stationing of allied troops here.