American Express Co., in a surprise move, yesterday offered to pay $830 million to acquire McGraw-Hill, Inc., an information conglomerate that publishes, among other things, Business Week Magazine.

Late in the day, McGraw-Hill said management's reaction to the proposed takeover was "negative" and urged stockholders to do nothing until the company's board meets to "consider all the ramifications of the proposal."

The proposal is a generous one. American Express said it would offer $34 a share, for the 24.1 million outstanding shares of McGraw-Hill. McGraw-Hill did not trade yesterday, but closed at $26 on Monday. American Express closed at 31 1/2, up 1/8 from Monday.

American Express also proposed an alternative plan, combining cash and securities.

Although Roger H. Morley, president of American Express, is on McGraw-Hill's board, the publishing company had no indication of American Express' interest until Morley and chief executive James D. Robinson III visited McGraw-Hill chairman Harold W. McGraw Jr. Monday night.

McGraw, in a statement released late yesterday afternoon, said, "I am personally very disappointed in the way in which American Express has handled this proposal."

McGraw holds a large chunk of the company's stock, as does his brother.

Industry sources said that the McGraw family controls between 20 and 25 percent of the information company's stock.

American Express said it was "disappointed," that McGraw-Hill management opposes the acquisition. "But we remain convinced the proposal is in the best interests of and provides excellent opportunities for stockholders... We are optimistic about the outcome."

American Express credit cards and travelers checks are recognized the world over. But in recent years the company has faced increased competition from bank credit cards, mainly Visa and Master Charge. Both Visa and Master Charge plan to offer their own travelers checks in the near future.

About half of American Express's earnings come from the travelers check and credit card business, while the rest comes from the large Fireman's Fund Insurance Co. subsidiary and its international banking operations.

American Express has made unsuccessful acquisition passes at Walt Disney Productions and Book-of-the-Month Club within the last year or so.

A spokesman for American Express said the company's desires to diversify have nothing to do with its competitive situation.

But Herbert E. Goodfriend, vice-president for research at Loeb, Rhoades, Hornblower, said that "obviously any time a company takes a diversification step it is saying its capital can be invested more productively outside its current line of business."

Most Wall Street analysts have not been recommeding American Express stock actively for some time, in part because of the sensitivity of its overseas earnings to exchange rate fluctuations and in part because of the competition in the credit card and travelers check field.

Recently, dollar fluctuations have not hurt American Express's earnings, which were $262.1 million in 1977, on sales of $3.4 billion.

But Visa and Master Charge have been trimming the discount they charge merchants who accept their cards, forcing American Express to go along.

"American Express's card volume is going up," one analyst said, "but its margins are shrinking."

The entrance of the bank cards into the travelers check field portends further competitive pressure. Not only are Visa and Master Charge wellknown, which should ease acceptance of their checks, but banks which are prime outlets for American Express checks are also often members of Visa or Master Charge.

In other merger news yesterday:

Mattel Inc., the toymaker, said it has reached an agreement to acquire Western Publishing Co., of Racine, Wis., in a cash and stock transaction worth nearly $121 million. Western is the country's biggest publisher of juvenile books.

The Federal Trade Commission has no objection to the proposed merger between Olinkraft Inc. and Johns-Manville Corp., and the two companies said they plan to complete the merger on Jan. 19.