Arab decisions at Fez could undercut Camp David plan.

US experts who see some merit in Saudi Arabia's ''set of principles'' for Middle Eastern peace view with trepidation this month's Arab summit meeting in Fez, Morocco.

''The worst thing that could happen,'' says a top US expert, ''would be for the Arabs at Fez to lock the Saudi plan into concrete, leaving all the sharp edges sticking out.''

Israel in that case might find it easier to draw the United States into open opposition to the Saudi initiative, which Israeli officials label a blueprint for Israel's eventual destruction.

''The question is,'' says the US expert, who has worked with Arabs and Israelis most of his career, ''how do you move from the Camp David process to a broader approach to peace?''

In that light, he sees the Saudi initiative - advanced as an eight-point peace plan by Saudi Crown Prince Fahd last August - as ''a positive and useful tactic'' that might lead to ''an eastern Arab negotiating base.''

Until now Israel has faced a solid wall of hostility from its eastern Arab neighbors - Syria, Jordan, Iraq, and Saudi Arabia - intensified by the Jewish state's peace treaty with Egypt, its Arab neighbor to the west.

Most Arab leaders interpret that treaty as an Egyptian device to regain lost Egyptian territory (the Sinai Peninsula), at the expense of the right of Palestinian Arabs to an independent state of their own on the Israeli-occupied West Bank and Gaza.

The late President Anwar Sadat also insisted on the ultimate right of Palestinian Arabs to sovereignty. But the Camp David negotiations, which he conducted with President Carter and Israeli Prime Minister Menachem Begin, settled for interim ''autonomy'' talks, so far stalled.

King Hussein of Jordan, in the US view, was powerless to extend peace feelers on his own, despite the benefits a wider Arab-Israeli peace would confer upon his Hashemite kingdom.

Now comes the Saudi initiative, which at least gives eastern Arab leaders a rallying point around which to discuss a potential peace process with the Jewish state.

''The substance of Fahd's eight points,'' concedes the US source quoted above , ''is not fully consistent with the Camp David approach. What is significant is the fact of Fahd's having made an initiative, which could lead to engaging other Arabs in a broader process.''

You need, he says, ''an eastern Arab base to solve the West Bank problem.''

Awareness of this need, according to sources interviewed, lay behind relatively positive initial references to the Fahd plan by President Reagan and other US officials.

''In fact,'' says one source, ''King Hussein heard on his Washington visit of an American commitment to a wider peace approach.''

Officials and other Mideast experts now foresee three dangers that could nip the emerging process in the bud:

* Hard-line Arabs at the Fez summit might cast the Saudi approach in terms so apparently threatening to Israel that the United States would find it impossible to explore this path further.

* British and other European leaders might muddy the issue by promoting a joint European-Saudi peace effort at the expense of the unfolding Camp David approach.

US officials, including those who see merit in the Saudi initiative, insist that the Egyptian-Israeli process must be allowed to work itself out, as a first step toward whatever wider peace efforts may develop.

* Israel, with its fears already aroused over the impending US sale of AWACs aircraft to Saudi Arabia, might prove unyielding on Palestinian rights on the West Bank and Gaza, undermining the Camp David process as a whole.

''The very fact of the Saudi initiative,'' says a veteran US diplomat, ''is of prime importance. Having the Saudis involved in the peace process is positive , even though they may not play it smoothly.''