When Chinese professors danced on a basketball court here Tuesday night, the happy tears of the onlookers made it seem more like a family reunion than the opening of the 11-day International Ballet Competition.

A group of eight teachers and dancers from the Peking Dance Conservatory, in America for the first time, performed a master class of Chinese folk dances which most of the more than 70 youthful IBC competitors and their coaches had never seen.

Those who had - including Chiang Ching, now director of her own dance company in New York, who had studied at the Conservatory - wept for joy that the People's Republic of China and the rest of the world can now dance together.

Six teachers from the Conservatory and two principal dancers of the National Dance Company came to exchange ideas, but will not compete. They will repeat their lecture demonstration in Jackson on June 27.

The Chinese group will spend two weeks in Jackson during this "world Olympics of Dance" - the first to be held in the United States - then go to New York and on to Durham, N.C. for the American Dance Festival which begins July 6. The tour was arranged by Chiang.

The master class introduced traditional dances of the Chinese provinces. Professor Jia Zou-Guang, won the greatest acclaim for his dance of the white goose. A duet, the dance of the eagles with Mme. Xu Su-Ying, brought the sparse audience - including Robert Joffrey, director of New York's Joffrey Ballet and dance critic Walter Terry - to its feet in a standing ovation.

The master class was open to the public, but most of the public didn't come to the Jackson State University basketball gym.

In this city of 250,000, an event of international cultural significance doesn't have to compete for attention. But the population seems hardly to know what to do with it. Only the 2,000 tickets for the final performance on June 29 have been sold out.

The city is little affected by its visitors, whose identity is unmistakable. The shape of ballet is everywhere: wraith-slim dancers with legs bulkier than their bodies go about town in leotards and warming socks despite Mississippi's muggy heat.

The competitors from the U.S. and 15 foreign countries rehearse all day and all night around the city - in the auditorium, the arts center, two high schools and three colleges. They move about town in vans and school buses.

Mississippi management blames slow ticket sales and slim attendance on 11th-hour arrangements. The IBC contracts were signed in January and arrangements made for the visiting Chinese group only last month.

Jackson's population is comparable to that of Varna, Bulgaria, where the IBC is held in even-numbered years. Moscow and Tokyo also have been hosts to the competition.

The first round of competition began yesterday, before an international panel of 13 judges from 13 nations. A gold medal and $7,500 is the top prize for the best man and the best woman dancer.

A total of 12 medals will be given in the senior division, six medals in the junior division, three medals for choreography, all with accompanying cash prizes plus some special prizes to be awarded next week.

Jury co-chairmen are Robert Joffrey and Sophia Golovkina of the Bolshoi Ballet in Moscow, Joffrey also will chair the jury for the 1981 IBC in Moscow. Golovkina judged previous competitions in Moscow and Varna.

Among the contestants claiming the most attention are two Americans: previous gold medal winner Jessica Funt and the youngest competitor, 13-year-old Deirdre Carberry of New York. Czechoslovakian dancers Lubomir Kafka and Jana Kurova are veterans of four international competitons and both are previous gold medal winners. Clotilde Vazer of France and Belgium's entry Koenraad Onzia also have won the gold before. There are no Russian dancers competing.

All the dancers will compete in the first round which continues through Friday. Thirty dancers will go to round two on the weekend contest held nightly Saturday and Sunday at 8 p.m. in the city auditorium. The finals are set for Monday and Tuesday nights.

Results and the winning performances of the nine gold and silver medal winners are scheduled for Thursday. In the final performance on Friday, June 29, all 15 medal winners will dance numbers not previously seen in competition.

Competitors and their coaches were offered free accommodations in a local college domitory, but all paid their own way to the competition. CAPTION: Picture, Jia Zou-Guang doing the "White Goose Dance"; by Laura Lynn Fistler