The invention discloses a cabin cover opening mechanism of a diving cabin at fixed depth in water. The problem that in the water, the door of a sealed cabin is easy to fall off is solved. The cabin cover opening mechanism comprises three stand columns (3) and a herringbone support (4), wherein the stand columns (3) are arranged on an annular water inlet hole base (2) at equal intervals in a cambered mode. A shaft sleeve (5) and a movable shaft (6) are fixedly arranged in the center of the herringbone support (4), the bottom end of the movable shaft (6) is connected with a cabin cover (8) through a connecting thread (14), a tension spring (15) is arranged between a top end cover (7) of the shaft sleeve (5) and the upper end of the movable shaft (6), a limiting hole (11) is formed in the side wall of the movable shaft (6), a pin puller hole (10) is formed in the herringbone support (4), and the head of a pin puller (9) is connected into the limiting hole (11) through the pin puller hole (10) in an abutting-against mode. The cabin cover opening mechanism protects an instrument in the cabin from damage and reutilizes the cabin cover.