The invention relates to a lock for a security door. The lock is characterized by comprising a lock shell with a containing cavity, a left sliding plate, a right sliding plate, an output gear, a clutch gear, a handle shaft and a speed reduction gear set. A mounting frame with a mounting shaft is arranged in the containing cavity, the left sliding plate and the right sliding plate are restrained onthe back face of the lock shell, a left rack part is arranged on the left sliding plate, a right rack part is arranged on the right sliding plate, the output gear is arranged on the mounting shaft ina sleeving manner, exposed out of the back face of the lock shell, and engaged with the left rack part and the right rack part in a transmission mode; the clutch gear is arranged on the mounting shaft in a sleeving manner and exposed in the containing cavity, and a clutch structure is arranged between end faces, close to each other, of the clutch gear and the output gear, the handle shaft is movably inserted in a shaft hole of the mounting shaft from the back face of the lock shell, the speed reduction gear set is arranged in the containing cavity, and the handle shaft is in transmission connection with the clutch gear through the speed reduction gear set. The lock for the security door is modular, the lock is suitable for a small-angle rotating handle, and also suitable for a 360-degreerotating handle, in addition, when a lawbreaker forcefully rotates the handle, the handle can idle, and safety is better.