The Right Rev. William B. Spofford Jr., 58, will become the assistant bishop of the Episcopal Diocese of Washington early next year, it was announced here today during the church's national general convention.

Spofford, who is resigning after 10 years as missionary bishop of the Diocese of Eastern Oregon, will aid Washington diocesan Bishop John T. Walker in the administration of church affairs here.

The employment of an assistant bishop is a relatively new strategy in the Episcopal Church. It enables the head of the diocese to select a man with talents to supplement his own.

The alternative -- and the practice of most dioceses -- is to call a diocesan convention to elect for life a man who is termed a suffragan bishop.

At the annual diocesan convention last January, Walker announced his intention of selecting an assistant bishop to share the burdens of administering the 63,000 member diocese. Walker has been the only bishop here since the retirement two years ago of Bishop William F. Creighton.

Spofford was the choice of a special screening committee appointed by Walker and chaired by David B. Beers, chancellor of the diocese.

When he begins his duties next February, Spofford will assist Walker with official visitations to parishes and with the pastoral care of clergy and their families.

In the Episcopal Church, such rites as confirmation, ordination of men and women as deacons and priests and the consecration of a church must be performed by a bishop.

Spofford, a native of Brooklyn, N.Y., has served churches in Boston and Detroit and was dean of St. Michael's Cathedral in Boise, Idaho.

Trained in pastoral counseling he has been supervisory chaplain and chief of pastoral services at Massachusett's General Hospital in Boston. He has served in a number of regional and national denominational posts including heading the Episcopal League for Social Action and as a staff member of the National Town-Country Church Institute in Missouri.

He is chairman of the Episcopal Church's General Board of Examining Chaplains, a post he will continue to hold after he moves to Washington.

He was appointed by the church's House of Bishops to the developing eastern Oregon diocese in 1969 several years ago he announced he would resign after 10 years of service to allow the diocese to choose its own leader.