When Egyptian President Anwar Sadat returns from vacation after the Moslem holy month of Ramadan, one of his first official acts will be an inspection trip to one of the most ambitious and politically sensitive construction projects in the Middle East - a motor vehicle tunnel under the Suez Canal.

The tunnel is to be the first land link between the Sinai Peninsula and the rest of Egypt since the destruction of a railroad bridge in 1967. It has been designed to carry two lanes of traffic as well as water and electricity lines for agricultural and industrial projects planned for the Sinai side of the canal. But it could also carry tanks and other military vehicles at a depth that is intended to make it impervious to Israeli air attacks.

Some 400 men are at work on the tunnel, which is scheduled to be finished in 1979. Named for Ahmed Hamdi, a "martyr" of the 1973 crossing, it is to be the first of two or possibly three. It is about five miles north of Suez city, near the southern end of the canal and the road to the strategic Mitla Pass.

Most of the Sinai Peninsula has been under Israeli military occupation since the 1967 war. But ever since Egyptian troops crossed the canal on pontoon bridges in the 1973 war. Sadat has been promoting plans for the development of the Sinai under eventual Egyptian control.

The construction of the tunnel, the widening and deepening of the canal itself, the rebuilding of the war-devastated cities on its western bank and the planned oil explorations in the southern Sinai are all major investments that are cited by the Egyptions as proof that they really do want and expect peace with Israel.

Egypt, stripped for revenues, simply cannot afford to have all that blown away again in a military conflict.

The Sinai itself is mostly desert, but the Egyptians hope to find manganese and coal as well as oil there. They say that a vast area has been identified by space satellite exploration as potentially arable.

If none of those plans works out, or if negotiations fail and Egypt does not recover the Sinai, the tunnel could become a $90 million white elephant.

But if war should come, the tunnel could be an important: asset to the Egyptian armed forces, a possibility that is known to have occurred to military planners in Israel.

It is wide and high enough to accommodate tanks armored personnel carriers and mobile missile launchers. It is no secret that the tunnel has, in the words of project engineer Ibrahim Zaki Kennawi, "military and strategic purposes."

He said the tunnel would withstand any non-nuclear air attack because of its depth - 150 feet below the bed of the canal, and 240 feet beneath the surface of the water.

Underwater tunnels are rare in this part of the world, and the Egyptian project, while not spectacular by American standards, will far exceed anything else in the Middle East.

The total length, with access roads, is to be nearly three miles. The covered portion will be 5,979 feet. (By comparison, the Baltimore Harbor Tunnel is 7,650 feet). An Egyptian construction firm is doing the work, with British technical assistance.