SEATTLE (AP) _ Boeing Commercial Airplane Co. today announced an order for six 737-500 jetliners, which helped the aircraft manufacturer break the industry record for new plane orders in a single year.

The order by Linjeflyg, a Swedish domestic airline, sent Boeing's orders for the year to $20.7 billion for 466 planes, breaking the Seattle-based company's own record of 461 orders set in 1978.

Linjeflyg's order, including spare parts, was valued at $175 million. The airline, a new customer for Boeing, also took out an option for six more 737s. The twin-engine jet is designed to carry 108 passengers.

John Hayhurst, vice president of marketing for Boeing Commercial Airplanes, said the outlook for further strong orders for Boeing is good.

Boeing is predicting that airline traffic growth will grow by 5 percent a year through 2005, when it will be about 2 1/2 times busier than today, Hayhurst said.

Future traffic growth translates into a need for 7,400 new airplanes between now and 2005, said Hayhurst, who added Boeing is well-positioned to get a good share of those orders.

Both GPA Rolls Ltd., of Shannon, Ireland, and Delta Air Lines of Atlanta have said they may be interested in ordering large numbers of Boeing planes, possibly this year.

''With the possible order of up to 50 757s pending with GPA Rolls, a Delta order and another possible 75 jets which could be used in the Navy's long- range anti-submarine warfare patrol aircraft program, Boeing could keep going like gangbusters,'' said Paul Nisbet, an aerospace analyst with Prudential- Bache Securities Inc. in New York.

Chris Longridge, vice president of sales at Boeing Commercial Airplanes, said today's order gives Boeing a commercial jet backlog of 1,036.

''This is the highest backlog we have ever had at Boeing'' and is the highest in the industry, Longridge said.

Presently at its Renton, Wash. plant, Boeing is turning out 14 737s and four 757s every month. The production of 757s will increase to five a month next February, Longridge said.

At Boeing's Everett plant, five 767s are being produced a month and three 747s, but plans are to boost that rate to five of the jumbo jets by mid-1989, Longridge said.

He declined to say whether these plans mean additional hiring at Boeing.

Boeing employment in the Puget Sound region currently stands at about 92,000.