The administration's plan to eliminate the Bureau of Alcohol, Tobacco and Firearms and spread its functions among other agencies would mean fewer inspections of gun dealers and widespread deregulation of the liquor industry, the Treasury acknowledged yesterday.

The move needs approval by Congress, which thwarted the ATF phaseout last fall.

Under the plan, the Secret Service would become responsible for enforcing the nation's firearms and explosives laws while the Customs Service would enforce laws relating to alcohol and tobacco.

Assistant Treasury Secretary John Walker said yesterday the administration also is asking that the budget for those duties be cut from the 1981 level of $150 million to $121 million for the 1983 year.

The spending cuts would mean a reduction of 983 jobs from the current authorized level of 3,433. ATF has already lost 376 positions during fiscal years 1981 and 1982. Walker said 1,200 agents and about 530 support people would go to the Secret Service and 720 employes to Customs.

The cuts will all but eliminate ATF's random inspections of gun dealers, Walker said. These inspections were aimed at making sure dealers were complying with the law and keeping adequate records of where their guns come from and to whom they are sold.