The Securities and Exchange Commission today charged a former chairman and the current senior vice president of Atari Inc., a unit of Warner Communications, with trading on the basis of illegal insider information just before Warner's announcement last December that its earnings would be lower then expected. Warner's stock fell $16.75 a share on the first day it traded after the announcement.

The agency charged that Raymond E. Kasser, the former chairman and chief executive of Atari, sold 5,000 Warner shares on Dec. 8, 23 minutes before the company issued a press release announcing that its 1982 results would be ''substantially below expectations'' due in large part to disappointing Atari sales.

Agrees to Give Up $81,875

Mr. Kasser, without admitting or denying the S.E.C.'s allegations, agreed to give up $81,875, representing the loss that he avoided by selling his stock before Warner's unexpectedly negative earnings announcement drove down the price. The money, S.E.C. officials said, will be turned over to the Federal District Court in San Jose, Calif., near Atari's head office. The court will allocate the money among investors who bought Warner Communications shares on Dec. 8. ''I seek to conclude,'' Mr. Kasser said in a statement released by the S.E.C., ''what otherwise promised to be a lengthy, costly and distracting litigation.'' He added that the sale of stock ''was arranged to fund a year- end tax shelter.'' He said, ''In retrospect, however, its timing was no doubt ill advised and creates a clear appearance of impropriety.''

Dennis Groth, senior vice president of Atari, who was also charged with trading on the basis of nonpublic information, denied the S.E.C.'s allegations, and said in a statement that he would vigorously defend against the insider trading suit.