China called Monday for an international fund to help the Third World join the fight to save the Earth's protective ozone shield.

Developing countries urgently needed cash and technology to begin using substitutes for ozone-destroying chemicals, Chinese Environment Commissioner Liu Ming Pu told a 120-nation conference.But the Soviet Union urged further investigation to discover whether depletion of the ozone layer, which screens out cancer-causing ultraviolet sun rays and helps regulate the planet's climate, was due to man-made pollution.

"The developed world's accumulation of a great amount of wealth was accompanied by the pollution and destruction of the environment," Liu told the conference, which opened Sunday.

"Now these countries can use past accumulated wealth to manage the environment . . . such is not the case for the developing countries."

Liu said the proposed fund should provide free transfer of technology to developing countries. India and Mexico also called on industrialized nations to help.

Delegates at the meeting have repeatedly urged participants to sign the 1987 Montreal Protocol, which calls for a 50 percent cut in the use of chlorofluorocarbons (CFCs) by 1990.

CFCs, used in aerosols, refrigerators, air conditioning and computer chip solvents and found in some medicines, are widely held by scientists to be responsible for eating a huge hole in the ozone blanket over the Antarctic.

European Community environment ministers agreed last week to ban the chemicals by the end of the century.

But EC officials concede it will be difficult to persuade poorer nations to forego the benefits of the durable, inexpensive and highly versatile CFCs which industrial countries have used for more than 40 years.

The Soviet Union, however, expressed reservations about extending the Montreal agreement without definite proof that ozone depletion was entirely due to man-made pollution.

Vladimir Zhakharov of the State Committee of Hydrometeorology said a fluctuating ozone thinning found over Eastern Europe was thought to be caused by atmospheric changes.