The Reagan administration has approved $49 million worth of high-techology sales to India in the past month and is seriously considering allowing it to buy a supercomputer in the first tangible signs of a closer relationship that has developed since the June visit here of Prime Minister Rajiv Gandhi.

The sales, involving the highest technology ever allowed to pass to India under U.S. export control laws, were part of a blitz of approvals over the past few months that enabled India to buy $894 million worth of sophisticated American products in the fiscal year that ended Sept. 30. This is nearly $400 million more than it was allowed to buy a year earlier.

This new relationship has also extended to the military side. During a visit in August by a high Defense Ministry official, India was given the go-ahead to buy an advanced jet engine from General Electric Corp. that would allow the Gandhi government to build its own combat planes and ease its dependence on Soviet-made MIGs.

"There has been a palpable difference in the past six months" in the Reagan administration's attitude toward India, with the White House taking the lead, said one ranking administration official.

"The pretty strong antagonistic view has done an almost 180-degree turn," the official added. "There's no question that Rajiv Gandhi's visit turned it around. He made a good case and all of a sudden the White House discovered India is an important country. It is no longer in vogue to have a hard-nosed attitude on India."

A sign of India's new status is the serious consideration administration officials are giving to the Gandhi request to buy a supercomputer for research on the monsoons.

There are fewer than 200 supercomputers in the world outside the U.S. national security establishment, and for India to get one would be a symbol of its emergence into the first world of scientific research as well as of its new relationship with the United States.

Export control laws currently impede the transfer of technology as sophisticated as supercomputers to nonaligned nations such as India. There are concerns, furthermore, that India could divert the supercomputer from weather research to defense applications, including the design of nuclear weapons.

Nonetheless, said administration sources, India's request has not been denied out of hand even though there is strong opposition from elements within the Defense Department who fear the technology could leak to the Soviet Union because of India's military supply and economic ties with Moscow.

"It is no longer an absolute 'no' on supercomputers," said one administration official.

"India will get it in time," another official declared flatly. "The point is, this purchase would never have been considered before."

Gandhi is considered likely to press India's case for a higher degree of technology, including the supercomuter, when he meets President Reagan at the United Nations next week.

Among the issues likely to come up during the Gandhi visit is the unresolved question of allowing India to make 600 small, entry-level computers over an eight-year period under a pending $500 million technology transfer agreement with Control Data Corp. The deal is hung up over the U.S. insistence that India give assurances it will not use the technology in its nuclear program. While the Gandhi government gave those assurances on equipment it is buying, it is reluctant to do so for computers that will be put together in India.

With a French company offering a similar arrangement, there is an Oct. 31 deadline for concluding the deal. State Department sources were optimistic the differences are likely to be resolved before then.

The high-technology sales were approved for India under a memorandum of understanding signed in May when Commerce Secretary Malcolm Baldrige visited New Delhi.

U.S. and Indian sources said a logjam over the implementation of the agreement for the highest level of technology was cleared in August, when V. S. Arunachalam, science adviser to the Defense Ministry, complained forcefully to White House National Security Advisor Robert C. McFarlane and Defense Secretary Caspar W. Weinberger.

During his visit, Arunachalam was taken in an Air Force jet on what one Pentagon official described as a "window shopping expedition" and shown a level of U.S. military technology never before seen by an Indian defense specialist.

It was during that trip that Arunachalam received a special security clearance to visit the GE factory in Lynn, Mass., and was provided with classified data on the F-404 engine.