NEW YORK (AP) _ Robert A. Beck, who has been chairman and chief executive of Prudential Insurance Co. of America since 1978, announced Tuesday that he plans to retire in February.

The board of the Newark, N.J.-based insurance and financial services company elected Robert C. Winters, vice chairman, to succeed Beck as both chairman and chief executive. An announcement was released in New York.

The statement provided no explanation for Beck's decision, but spokesman Rick Matthews described it as ''purely voluntary.''

Beck turns 61 next month, and his scheduled retirement next Feb. 5 coincides with his 36th anniversary with Prudential. He joined the company as an insurance agent in 1951.

Since Beck took over as chief executive in 1978, the company's assets have risen to more than $120 billion from less than $50 billion, and it has another $40 billion under management.

Winters, 54, joined Prudential in 1953, and was elected vice chairman in 1984. He has been in charge of the central corporate and financial operations.

A. Douglas Murch, 57, senior vice president and chief actuary, was named executive vice president, assuming most of Winters' current responsibiliti es, the announcement said.