The invention discloses a standby water inlet detection device for a watertight cabin and a detection method thereof. The standby water inlet detection device comprises a detection bolt, a bolt seat,a circular nut and a hexagonal thin nut. The circular nut is arranged on the inner side of a cabin wall plate. The bolt seat penetrates through the cabin wall plate to be distributed on the two sidesof the cabin wall plate and comprises a boss, a connecting end and a through hole; the boss is arranged on the outer side of the cabin wall plate; an outer thread is arranged on the outer wall of theconnecting end, an inner thread is arranged on the inner wall of the connecting end, and the connecting end is in threaded connection with the circular nut through the outer thread; and the detectionbolt penetrates into the through hole to be in threaded connection with the connecting end of the bolt seat. A sealing cushion is arranged between the boss and the detection bolt; an axial detection through hole and a radial detection opening are formed in the detection bolt; one end of the axial detection through hole communicates with the radial detection opening, and the other end of the axialdetection through hole communicates with the internal space of the cabin wall plate; and the distance a between the left side wall of the radial detection opening and the end of the detection bolt isnot smaller than the thickness b of the sealing cushion. The detection measure is safe and reliable, and the operation safety of workers can be improved.