The utility model relates to the field of drilling machines, in particular to a normally-closed safety gear device of a petroleum drilling machine hydraulic disc brake. According to the technical scheme, the normally-closed safety gear device comprises a hydraulic cylinder piston 8, a disc spring 12, a threaded rod 13, a threaded rod 13, a female thread body 17 and a lock nut 18. The threaded rod 13 and the female thread body 17 compose a thread pair in telescopic rotating fit, and the lock nut 18 and the threaded rod 13 are locked and matched in a threaded mode. The disc spring 12 is arranged between the hydraulic cylinder piston 8 and the female thread body 17, and a disc spring pressure indication mark 9 is marked on an extending-out arm of the hydraulic cylinder piston 8. According to the technical scheme, the normally-closed safety gear device further comprises a shell 10 which is arranged between the hydraulic cylinder piston 8 and the outer connecting end of the threaded rod 13. According to the normally-closed safety gear device, the torque is adjusted to be moderate, adjustment of the thread pair can be effectively protected, when the gap of the disc brake is adjusted, the female thread body can be rotated according to the locking indication mark, and the pressure of a brake block is adjusted.