WITH THE SIGNING of the U.S.-Chinese nuclear agreement, its text will at last become public. The long delay has ended, and the administration has now decided to go ahead with the agreement while China's president, Li Xiannian, is here. When it reaches Congress, it is likely to become the focus of a long and careful examination of the intricate system of rules that try to prevent the proliferation of nuclear weapons. The agreement would enable China to buy civilian power reactors here -- on condition that it gives no assistance to other countries trying to build nuclear weapons.

This agreement was initialed 15 months ago during President Reagan's trip to China but, after his return, slid silently into the deep freeze. The administration has never offered a public explanation,it appears there were intelligence reports of Chinese technicians' working at Kahuta, where Pakistan has been building a uranium enrichment plant with technology stolen a decade ago from a similar facility in the Netherlands. The Chinese then disappeared from the Pakistani nuclear plant, and the Chinese government repeatedly declared that it does not help other countries to make weapons.

That now presents the United States with a choice. One side of the argument is that China has never offered the kind of detailed and carefully defined assurances that meet American legal standards. The other side is that it's well worth securing even the Chinese style of pledge from a country that already possesses both nuclear weapons and a good command of nuclear technology -- and that had previously been unwilling to offer any pledge at all.

Congress can block the agreement by a joint resolution. That's unlikely, but there are two kinds of congressional concern that the administration is going to have to address. A number of senators, most of them Democrats, are going to press for assurances that the agreement meets the letter of the American nuclear export laws that were enacted to prevent the proliferation of weapons. There are also objections, originating in the Defense Department, that even civilian nuclear technology has, in the Chinese context, military applications. They have less to do with weapons directly than with the development of nuclear power plants for naval ships, particularly submarines.

The administration bears the burden of demonstrating that this agreement, inadequate a year ago, is satisfactory today. That is not an impossible case to make, but the administration is going to have to make it openly and forcefully. A lot depends on the precise language of a text that no one outside the administration has yet seen. But at least potentially there may be important benefits here for controlling the spread of nuclear weapons. Until recently China chose to remain entirely outside the structure of international promises that seek to prevent proliferation. An agreement with the United States, providing access to certain American technology, would provide a powerful incentive to come in and stay in.