Stephen M. DuBrul Jr., former chariman of the U.s. Export-Import Bank, said yesterday that if the bank is "thoughtfully administered" it can live within budget cuts proposed by the Reagan administration.

DuBrul, testifying before a House Banking Committee subcommittee, said the Office of Management and Budget "has gotten off to a good start" in devising budget reductions and that "Ex-Im should be able to do its job well within those new levels."

The administration has proposed cutting Ex-Im's budget for the current year by $1.1 billion to $4.9 billion and from $5 billion to $3.3 billion in fiscal 1982. That includes a reduction in new direct loan originations below levels planned for the next five years, a prohibition against interest subsidies in guaranteed loan transactions and elimination of discount loans.

The government agency uses low interest rates to help lure foreign customers to buy American-made jet aircraft, turbines, nuclear reactors and similar high-priced items. The bank's loans recently have carried interest charges of 8.5 to 9.5 percent, but the bank then borrows loan funds from the Treasury at a cost of 10 to 12 percent. The difference is made up from the bank's budget or from cash subsidies from the Treasury.

Since 1977 the bank's export financing has grown by nearly 600 percent and has been channeled increasingly to big U.S. aircraft and power generating equipment manufacturers such as Boeing Corp., Westinghouse, McDonnell Douglas, Combustion Engineering and Lockheed.

The Boeing Corp. uses 42 percent of Ex-Im direct loans that support aircraft sales, and its chairman, T.A. Wilson, testified that his company stands to lose billions of dollars in sales in cases in which it comes head to head with subsidized foreign producers.

"It is difficult to accurately predict the number of orders that will be lost or deferred if Ex-Im financing participation is either not available or, if available, not competitive," Wilson said. "However, our market projections indicate we will be competing for approximately $2.5 billion to $3.5 billion worth of orders in the balance of fiscal year 1981, which we believe will require Ex-Im participation.

"The loss of these orders in this time period could result in the loss of follow-on orders that over the next decade could approximate $10 billion," Wilson continued.

Slightly more than half of the billions of dollars the company expects to lose involve potential Boeing 757 and 767 orders that are directly competitive with planes made by Airbus Industrie, a major world aircraft producer that Wilson said is highly subsidized by France, Germany and Great Britain.

Boeing also said the loss of sales would mean a loss of American jobs in the aerospace industry. Boeing will survive the budget cuts, but some related industries and small businesses with which it works may not, Wilson said.

"I got three calls from Ex-Im people who said, 'Steve, we can do it,'" DuBrul said. "'We can do it if we work it out more thoughtfully . . . , if we're selective.'"