Melodrama in ancient China; Manchu, by Robert Elegant. New York: McGraw-Hill. $12.95

"Manchu" does for 17-century China what James Clavell's "Shogun" did for 16 th-century Japan. It plumps a handsome, self-reliant Englishman down in an exotic, alien setting and follows his tribulations and travels through a crisis-ridden country.

Against such a background, the full scope of history can come into play. Strikingly different cultures and customs clash. In addition to the basic conflict between European and Asian mores, there are vast internal squabbles racking China -- the waves of which, naturally, rock our hero.

He is soldier-of-fortune Francis Arrowsmith, known to the Chinese as Arrowmaker. His great talent is that he knows how to build and deploy European-type cannon, when Asian warriors still are pretty much confined to bows and arrows. (Why the Chinese didn't dub him Cannonmaker instead of Arrowmaker is a point I must have missed.)

At any rate, the plot allows Mr. Elegant to display his abundant knowledge of China and the Chinese language to great effect. The Peking of the 1600s comes to life in full color and meticulous detail. Various Chinese meals and their full ingredients are examined, as are the clothes people wear, the weapons they use, the roads they travel, the houses they live in, the ceremonies and customs that govern their lives. As do Michener and Clavell, Elegant teaches much about his locale -- occasionally a bit more than the uninitiated will want. For example, in the book Arrowmaker (who is a quck study) often speaks sentences of straight Chinese which, fortunately for most of us, are immediately translated.

Making narrative use of all this information is Robert Elegant's great talent -- the ability to take us within hailing distance of true history and real emperors, and yet weave an imaginative story around the fictitious characters he inserts. It is a device that served him well in his previous best seller, "Dynasty."

Arrowsmith-maker has happened on the scene at a stirring moment of Chinese history. The Old Ming Dynasty, corrupt and decadent, is about to fall apart. So Arrowmaker is persuaded to help the Mings with new artillery. He takes a Chinese wife, Marta, related to the prime minister. For a time it seems as if Francis and his remarkable machines may save the Ming court and its mandarins from the barbarian Manchu hordes at the northern gates.

But soon he is captured by the marauding Manchus. The master cannoneer is pressured to help them, again by building new artillery. He takes a tempestuous Manchu wife, Barbara. He accompanies the invaders as they capture Peking and gradually digest the rest of China.

Eventually Francis goes back to Portuguese Macao, a tiny colonial dot near Hong Kong whence he started his odyssey, to learn more of gun-building. There he makes an alliance with a third culture by marrying yet again -- this time the sophisticated, wealthy Portuguese, Dolores. At last, one feels, Arrowmaker has come home to his European heritage. But no, China still beckons, and Francis is soon off (despite the disapproval of Dolores) on another doomed military campaign, to relieve besieged Canton.

Alas, this vast epic at times may weary the reader by the sheer weight of its literary brocade. It starts slowly, and sorting out the multitude of Chinese officials and Jesuit missionaries can become a virtual Chinese puzzle.

But as its best, "Menchu" shows us ancient China at its most vivid. Warrior horsemen clatter across its pages, spears and arrows at the ready, sometimes with an early Chinese John Wayne at their head. Forts, cities, and empires duly fall. Courtiers plot intrigues. Courtesans exert their wiles. The silken cord takes its toll.

It is as if all the Chinese screens, paintings, on glass, and scrolls of yore have come to life, eclipsing by far the tiny human characters who stir around them.