A record budget deficit of $109.5 billion for the current fiscal year, nearly triple the target set by Congress last summer, was forecast by congressional budget experts yesterday as some House Democratic leaders made a preemptive strike against new cuts planned by President Reagan.

In another warmup for battles that will begin in earnest when Reagan submits his fiscal 1983 budget Monday, the House Appropriations Committee approved nearly $10 billion in supplemental appropriations for this year. Included are $2.3 billion in unemployment aid that the administration wanted and $123 million in low-income fuel assistance that it opposed.

The deficit prediction was based on Congressional Budget Office figures included by the House Budget Committee staff in the initial draft of a final budget resolution for fiscal 1982. The committee delayed action on the resolution until it receives deficit projections from Reagan, who said last week that his deficit forecast would not top $100 billion.

The budget committee staff attributed the huge spurt in the deficit projection to "unrealistic economic and other assumptions" that Congress wrote into its budget targets last summer at the administration's behest.

Instead of growing as the targets anticipated, the staff noted, the economy sank into recession, adding to government costs. High interest rates also added to costs, while the recession and declining inflation resulted in a sharp reduction in revenues.

The first resolution fell short of its revenue target by $26.6 billion and exceeded its outlay target by $45.2 billion, a deficit miscalculation of $71.8 billion, the staff said.

Congress is supposed to pass another version of the final 1982 budget resolution, which sets binding ceilings for spending, because its pre-Christmas vacation version was simply a carbon copy of its first resolution, with no adjustments for economic changes. Because of huge deficits Congress would have to sanction in passing another resolution, a vote for adoption is uncertain.

The attack on Reagan's 1983 budget came from the chairman and subcommittee chairmen of the House Commerce Committee, who charged the administration with "Draconian" spending cutback plans that would have "serious consequences for millions of individual Americans."

That committee oversees activities ranging from health to transportation and energy, and was more successful than most committees in withstanding Reagan budget cuts last year.

Claiming their information came from administration budget documents, the committee leaders said Reagan will seek a $5 billion reduction in health services, deep slashes in funds to control hazardous wastes, a massive shift from civilian energy needs to military nuclear programs and confinement of the Amtrak passenger rail system to the Northeast corridor.

Among specific cutbacks cited by committee chairman John D. Dingell (D-Mich.) were $2.2 billion in Medicaid payments to the states, a reduction of nearly one-fourth in funding for maternal and child health, a continuation of cuts for child immunization services, cutbacks in regulatory activities aimed at investment fraud and further reductions in energy assistance for the poor and elderly.

The supplemental appropriations bills included $5 billion for farm price supports, $2.4 billion for grants to build sewage treatment plans, $1.9 billion in loans to states whose reserves for unemployment benefits are running out, $343 million for employment services including money to fill jobs vacated by last year's budget cuts and $123 million for low-income fuel assistance.