The Henley Group Inc., a diversified holding company, said yesterday that it is negotiating to acquire Santa Fe Southern Pacific Corp. for about $9.5 billion in cash and securities.

Henley began increasing its investment in Santa Fe, which previously had begun a restructuring in hope of avoiding a takeover, when the stock market collapsed Oct. 19.

Henley, a La Jolla, Calif., firm that already owns 14.7 percent of the railroad and natural resources company, said in a filing with the Securities and Exchange Commission yesterday that Santa Fe proposed that Henley acquire it for $63 a share in cash. The two companies then began discussions of a cash and securities deal.

Acquiring the rest of the Chicago company would cost Henley about $8.38 billion, in addition to the approximately $1 billion it has already spent for stock.

The price under discussion is slightly below the breakup-value estimates of the company's worth. But Henley appears to be in a strong position to succeed with its offer unless another bidder emerges, according to analysts.

The company's attractions include profitable real estate holdings and oil and gas operations. Henley, which owns and manages a collection of manufacturing, technology and financial services companies, acquired a sizable position in Santa Fe -- 4.9 percent of the then outstanding shares -- in 1986 and first raised the prospect of acquiring Santa Fe in June.

On June 30, the Interstate Commerce Commission refused to reconsider its rejection of a proposed merger of the Atchison, Topeka and Santa Fe Railway with the Southern Pacific. The Santa Fe Southern Pacific Corp. had operated the two railroads separately pending the federal approval that never came.

Since then, Santa Fe has moved to reorganize itself and to sell the Southern Pacific, as required by the ICC, hoping to avoid becoming a takeover target itself. Last month Santa Fe announced that it had received bids ranging from $750 million to more than $1 billion for the Southern Pacific.

Santa Fe stock was up $4.75 yesterday to $55.