A 36-year-old Washington man pleaded guilty yesterday in D.C. Superior Court to second-degree murder in the beating death earlier this year of his 17-month-old son.

The incident occurred three weeks after the infant had been returned to his parents' home from a foster home where he had been placed following an earlier report of mistreatment.

After Jerry H. Wells Sr. pleaded guilty to the murder charge, Judge Peter H. Wolf ordered him held without bond pending sentencing Oct. 27.

Wells, who faces a sentence of 15-years-to-life in prison, had been convicted in late 1980 of assaulting the infant, assistant U.S. Attorney Brooks Harrington told Wolf at the hearing yesterday.

Wells, who had worked as a custodian at the Smithsonian Institution, was placed on probation and ordered to seek psychiatric treatment after that incident, Harrington said, and the infant, Jerry H. Wells Jr., was placed in a foster home.

Harrington said city social services workers returned the child to his parents in mid-December 1981.

Three weeks later, while the child's mother was at work and Wells was home alone with his son, Wells called his wife and said the boy was sick, Harrington told Wolf. But when the mother returned home about one-half hour later, the child was dead.

An autopsy by Deputy D.C. Medical Examiner Dr. John Hunsaker showed that the boy died of a fractured skull and that the child appeared to have been hit more than 20 times, apparently with a fist, Harrington said.