Sen. Gaylord Nelson (D-Wis.), regarded by many as the father of the modern environmental movement, next month will become chairman of the Wilderness Society, a pro-conservation lobbying group.

Nelson, 66, was narrowly defeated last month for a fourth Senate term. He previously was a state legislator and two-term governor of Wisconsin.

He said yesterday that he will join the society Jan. 14, overseeng policy and strategy planning, congressional relations and resource evaluation work.

The Washington-based Wilderness Society focuses on use, protection and management of public lands.It is supported by dues from 42,000 members and by foundation assistance.

In Nelson, the society has landed one of the country's best-known environmental figures, the creator of Earth Day and author of some of the principal resource protection laws of the past 15 years.

Among them are statutes on wild and scenic rivers, hiking trails, eastern wilderness, DDT pesticide limitation, water pollution control, auto fuel efficiency and national environmental policy.

Nelson will join the society at a time his environmental colleagues are fearful of a wholesale assault or air, water and public lands protection laws by the incoming Reagan administration.

Concern has heightened in recent days with indications that James G. Watt, a conservative critic of tough environmental standards, will become secretary of interior.

Nelson predicted yesterday that any general attacks on environmental laws would not last long, simply because "it would be a political impossibility . . . the public will recognize it is fundamentally wrong."

"I'm concerned by what I read and see about the new administration, but I don't see any long-term reversal of the environmental gains of the last 20 years," he said.

"I hope President-elect Reagan recognizes the compelling necessity for resource management programs that were supported by his Republican predecessors."

Nelson said environmental concerns have become so commonplace that virtually all senatorial offices have full-time environmental specialists, another factor that makes policy reversals politically difficult.