The invention relates to the field of seabed immersed tunnel floating construction and provides a temporary passage above a ballast tank and a passage mounting method. The temporary passage comprises triangular supports distributed along the passage direction of a carriageway, the triangular supports are arranged above the ballast tank, one end of the tripod is fixed on the side wall of the carriageway, the adjacent triangular supports are connected through a first cross brace in the horizontal direction, pedals are arranged on the first cross brace in the horizontal direction, a guiderail is mounted outside the pedals and connected with the triangular supports and is provided with a passing port, and the guiderail is connected with a ladder through the passing port. By the temporary passage, the carriageway can be kept smooth passing, and subsequent construction is facilitated. On this basis, the temporary passage is stable and durable, and convenient and quick to construct and demounting and can be mounted and demounted repeatedly. Furthermore, the temporary passage is small and convenient to transport and store after demounted.