The invention discloses a limiting and automatic-unlocking mechanism for a rotation shaft system. The limiting and automatic-unlocking mechanism is applied to the field of the space technology and aims to solve the problems that existing limiting and unlocking devices for the rotation shaft system are complex in structure and low in reliability. The limiting and automatic-unlocking mechanism mainly comprises a limiting gear, a guide support, a limiting guide rod, a first spring, a tightening cover, a steel ball, a clamping base, a second spring and a tightening screw in structure; the limiting gear is fixed to the rotation shaft system; the guide support is connected to a machine body through threads, the limiting guide rod is arranged inside the guide support and is guided by the guide support, and the lower end of the limiting guide rod is arranged on the limiting gear; the first spring is arranged at the upper part of the limiting guide rod in a sleeving mode; the tightening cover is connected to the upper part of the guide support through threads, so that the first spring generates pressing force; the clamping base is arranged on the outer wall of the guide support, and the steel ball is arranged inside the clamping base and is pressed by the second spring and the tightening screw. The limiting and automatic-unlocking mechanism is small in structure, adjustable in limiting and locking degree and capable of being widely applied to the field of the space technology and related rotation equipment.