The Arlington County Board was expected to give preliminary approval last night to plans for a referendum next spring on a $50 million bond issue to pay the county's share of completing the 100-mile Metro subway system.

Arlington is the first jurisdiction in the metropolitan area to act on proposed two-stage financial plan that Metro sent to local jurisdictions last August. However, the action could be reversed if the balance of political power shifts after the Nov. 7 elections.

For the past two months board chairman Joseph S.Wholey, who is also a member of the Arlington County Board, has been attempting to win support for the draft plan through a series of briefings for local officials.

U.S. Transportation Secretary Brock Adams has said that the Carter administration is committed to the goal of the 100 mile system but local governments such as Arlington's must first guarantee that they will pay their shares of construction and longterm operating costs.

Wholey and other Metro officials have repeatedly said in their briefings that should any jurisdiction refuse to pay its share, the system would "grind to a halt."

The plan Arlington was expected to endorse calls for finishing the system by 1985, with 80 percent of the $3.2 billion completion costs paid by the federal government. The remaining 20 percent would be paid by state and local jursidictions.

"This doesn't bind us to any referendum," said Anton S. Gardner, Arlington's fiscal analysis chief yesterday. "We're not going to go to any referendum until we have some reasonable assurance that other people (local jursidiction and the federal government) are with us."

Gardner said it is unlikely that a binding decision on a possible referendum would be made before next March, after local elections are over and "when we see the effect that tax cutting proposals similar to Proposition 13 have in Montgomery and Prince George's counties.

On Nov. 7 Arlington voters will choose Wholey's successor on the County Board. That election will also determine whether political power will remain with the Democratic coalition, as it has for the past decade, or shift to the Republicans.

A Republican victory could mean that Walter L. Frankland Jr., an outspoken critic of Metro and opponent of the 100-mile system might become chairman of the County Board.

Under the plan the board was expected to endorse fares for the bus-rail system that would keep pace with inflation, rather than rise at half the inflation rate, which is current practice.