When it comes to China policy (and perhaps some others), Ronald Reagan has the quality of a heavy-duty rubber band: he is elastic enough to be stretched right up to the point of accepting that the Carter administration's policy, with a few distinctive flourishes, is the only way to go.

This means moving, ever so delicately, toward closer connections of all sorts with Peking, most notably in trade involing goods and technology, with potential military application. It also means maintaining no more than "unofficial" relations with Taiwan, in keeping with the "normalization" agreements reached with the People's Republic of China in 1979.

But there is the ever present possibility that the president, if turned loose, would snap right back into the arms of the Taiwan lobby and the halcyon days of Chiang Kai-shek. We saw that happen in last fall's campaign fiasco, when candidate Reagan angered and alientated the Peking government with irrepressible promises to restore some sort of "official" relations with Taiwan.

According to people in a position to know, that same nostalgic tug was still very much in evidence among some of the president's closest confidants in the preparations for Secretary of State Alexander Haig's first official trip to China, which is now under way. It is one reason why Haig's probe of Peking may be his most challenging and personally precarious mission so far.

"One false step with the China issue," says one observer, "and the White House will be ready to pounce."

The path to a new and distinctively different Reagan China policy (by the rules of partisan politics, all of the adminstration's policies must seem, at least, to be distinctively different) presents no end of possiblities for false steps. To see why, it is necessary to understand the almost incredible precision required in the safe handling of the U.S.-China relationship.

It is, to begin with, a relationship calibrated very largely by symbols and appearances. In dealings with far smaller and less consequential friendly nations (with China, it is unforgivably presumptuous to use the word "alley"), U.S. economic and/or military aid may be measured by the hundreds of millions of dollars. Military aid comes in the form of tanks jet fighters, missiles.

In dealings with the world's most populous nation, American good will and support is measured almost not at all in sales of anything with a potential military use. The Chinese are avid window-shoppers, eager for technology, but financially incapable of buying in bulk. But they care a lot about the right to buy, or bid.

And so the test of American frienship is in our readiness to license purchases of items that, for example, we wouldn't make available to the Soviets but might sell, say, to Egypt or Yugoslavia. It's a matter of status -- and a measure of our trust.

Thus, back in 1978, the Carter administration took what was regarded as a giant step by authorizing for sale to China a relatively few items of so-called dual-use technology, with civilian as well as military applicability, that we had denied to the Soviets.

After the Afghanistan invasion, this "China differential" was pointedly enlarged by establishing a special category for China. This smoothed the way for the sale of dual-use technology as a general rule, rather than as case-by-case exceptions. At the same time, limited opportunities were offered for Chinese purchases of non-lethal military equipment: radar, trucks, helicopters, communications gear.

Not much has resulted from these moves. Whether this is entirely because the Chinese are in no hurry or partly because of bureaucratic snags is a matter of argument.

In any case, as we read of Haig carrying with him to Peking an adminstration decision to "sell" new kinds of technology to China or to permit sales of more military equipment, it's worth keeping in mind that this may amount to little more than an explicit commitment to comply with what has already been promised. In other words, it is not as "new" as it may be made to look, if only because any abrupt departures -- introducing lethal equipment, for example, threaten two kinds of instant instability.

One would be to overplay the "China card" in a way the Soviets might misread. There are some in the administration who would take this risk. But it is unlikely Haig will do more than test the market for lethal items like anti-tank missiles. The other obvious effect would be to inflame the Taiwan lobby, already clamoring for advanced American jet aircraft.

By most accounts, Haig is acutely conscious of the need for probing and tinkering -- but departing only superficially from the Carter policy, at least for now. He is equally conscious that even within the adminstration, this may not be an easy line to hold. "Haig seems to see himself," says one recent visitor, "as the sole rational defender of continuity in China policy."