The Navy has awarded an estimated $125 million in new shipbuilding work to the Bethlehem Steel Corp.'s financially battered Sparrows Point shipyard near Baltimore, assuring jobs for hundreds of laid-off workers during the next two years.

The announcement follows a recent Navy award of two other shipbuilding projects to the Sparrows Point yard. A spokesman for Rep. Clarence D. Long (D-Md.) said the three projects will provide $375 million in work and amount to the largest single contract ever awarded to the Sparrows Point yard.

A Bethlehem Steel spokesman said the shipbuilding work will help provide jobs for 1,900 workers at Sparrows Point from mid-summer to about 1985. Only about 850 workers are currently employed at the yard, the spokesman said. Even with the Navy projects, the yard's total employment will still fall short of its normal 3,000-member work force. "We're hoping to find additional work," the spokesman said.

Until recently, Bethlehem Steel operated two major shipyards in the Baltimore area in addition to a sprawling steel manufacturing plant at Sparrows Point, a peninsula southeast of the city. The company closed its ship repair yard, overlooking Baltimore's inner harbor area, on Dec. 31 and is negotiating in an attempt to sell it to another firm.

Its separately-run shipbuilding yard at Sparrows Point has been troubled by a falloff in orders from commercial shippers and oil rig businesses. In addition to the prolonged economic downturn, the yard has been hard hit by the Reagan administration's policy of encouraging commercial shipping firms to have their vessels built and overhauled in foreign yards. Last year, company officials warned that the yard faced possible closing unless new work was found.

The Navy's announcement represented a decision to exercise an option under a previous contract, signed with Bethlehem Steel in early November. The option provided for an additional $257,499 in shipbuilding work at the company's shipyards at Sparrows Point and Beaumont, Tex. The contract, including the option, now amounts to five projects valued at more than $600 million at both yards.

Officials said the work to be carried out at Sparrows Point will entail conversion of three cargo ships, owned by Maersk Line Ltd., a Danish company, into Navy vessels for the Rapid Deployment Force. The commercial ships will be lengthened from 597 feet to 754 feet, the Bethlehem Steel spokesman said. They will be outfitted with new decks and other structures to handle tanks, artillery, helicopters and other equipment needed in a military emergency.

The first Maersk ship arrived at Sparrows Point Dec. 29, the company spokesman said, and the others are due next November and in April 1984.