House and Senate conferees failed yesterday to reach agreement on how big a target for defense spending the premiminary 1978 budget should contain.

Staff members for both the Senate and House Budget committees worried that the conferres would be unable to agree on a compromise defense figure, although House Chairman Robert N. Giaimo (D-Conn.) said he was encouraged that they were still "talking."

House conferees declined to go much above the $117.1 billion in spending authority the Houe approved last week. The Seante conferees were not even sure they would approve a compromise offered by Sen. Henry L. Bellmon (R-Okla.) to bring down the Senate's level from $120.3 billion to $118.7 billion.

It did not matter, because the House conferees rejected the Bellmon offer out of hand.

The conferees wil meet again this morning to try to join out their differences. Ironically, the target over which they disagree is not a binding one. It can be adjusted up or down when Congres adopts a binding budget next September.

But neither the House nor the Senate can consider any spending bills until Congress agrees on a preminary budget. It is supposed to do so by May 15.

It has been a rocky road for the two-year-old congressional budget process in the House, where the first attempt to pass a preliminary budget was roundly defeated two weeks ago.

Liberals disapproved of the high defense spending level approved on the floor, and conservatives, most of whom voted for the defense increases, turned around and voted against the overall budget because the deficit was too large.

After heavy pressure from the House leadership, a compromise was fashioned last week that boosted defense spending a little, but not enough to lose liberal support. While the budget process has been a bipartisan one in the Senate, few Republicans ever vote for the House version.

The Senate version contains a deficit of $63.5 billion, but would have spent more on defense and less on other areas than the House. The House version contains a $66.4 billion deficit, but spends $5.2 billion more than the Senate, assuming $2.4 billion more in revenues.

On Monday the conferees agreed rather easily on compromise spending figures for all other areas but defense. They knocked $3.6 billion out of House spending totals, but only $1 billion represented real increases, with rest due to technical adjustments and re-estimates of spending needs.

Giaimo complained yesterday that he could not bring back a compromise to the house that cut $1 billion from social programs but adds several billion dollars to defense.

He told Senate Chairman Edmund S. Muskie (D-Maine) that 53 per cent of the House Democrats have served less than four years. "They are not anti-defense, but they are skeptical," he said.

There are certain "symbols" in the House, he said, "that make 53 per cent unhappy: when they see reductions in human programs, no new starts, a balanced budget for 1981 but warnings not to touch a penny for defense."

Muskie said that the Senate conferees could not go too far below the spending authority target of $120.3 billion without risking failure in the Senate.