The debate between the United States and Libya over an alleged chemical weapons plant may overshadow a 140-nation conference on the use of poison gas and other chemical arms. Such weapons have been called a poor nation's atomic bomb.

Secretary of State George P. Shultz and a Libyan delegate plan to attend the opening session Saturday, and tension is expected.Foreign Minister Roland Dumas of France said his government hopes the five-day conference will not "deviate from its agenda" because of the shooting down Wednesday of two Libyan jet fighters by U.S. Navy planes.

Delegates are expected to discuss changes in the 1925 Geneva Protocol prohibiting the use of poison gas and other chemical weapons. The document does not forbid making the weapons and has no enforcement provisions.

With their effectiveness, relative economy and ease of production, such weapons are feared because they greatly increase the killing power of smaller, less armed countries.

Many countries want to give the protocol teeth and spur negotiations on a treaty banning production of chemical arms. They include the United States and Soviet Union, the only countries that admit having the weapons.

Both French and U.S. officials indicated they would honor a "gentlemen's agreement" against name-calling at the conference.

The United States has been vocal in the weeks before the conference about what it says is a big chemical weapons plant Libya has built south of Tripoli and is ready to go on-line. Libya says the factory manufactures only pharmaceuticals.

President Reagan refused to rule out military action against the plant at Rabta. Washington said Wednesday's air action over the Mediterranean was not related to the allegations.

But Shultz said Wednesday that the United States may not have to level the plant if Libya is willing to work out a deal.

"Whether there is something else that can be done, we'll see," he told reporters on a flight to Paris from Washington. He did not elaborate.

The private Stockholm International Peace Research Institute says at least 10 countries are reliably reported to have chemical weapons. The United States says about 20 either have produced the weapons or are capable of doing so.

Iraq's use of mustard gas in its eight-year war with Iran and charges that it used poison gas last fall on rebellious Iraqi Kurds have kindled public interest in chemical weapons.

Reagan proposed the conference in his address to the U.N. General Assembly in September. France, as official keeper of the 1925 Geneva Protocol, offered to be host for the meeting.

President Francois Mitterrand will give the opening address when the conference convenes at the headquarters of the U.N. Educational, Scientific and Cultural Organization. U.N. Secretary-General Javier Perez de Cuellar also will speak.

French officials emphasized the conference is diplomatic in nature and will end with a communique that has no legal weight, but Washington and Moscow want action.

The United States wants to enhance the U.N. secretary-general's role in investigating allegations of chemical weapons use. It also will urge industrialized nations to deny such technology to Libya and approve economic sanctions on thosewho help proliferate use of the weapons.

Soviet officials seek an agreement requiring all nations, including the superpowers, to destroy existing chemical weapons over a 10-year period.

Mitterrand also has urged a sanctioning role for the United Nations. He also has called for an embargo on deliveries of products and technologies involved in the manufacture of chemical weapons and a further embargo on all arms to "nations in question."

The protocol is a one-page document with 118 signatories that has long been considered inadequate.