A mechanical automatic vertical drilling tool, with ends that are connected with an upper drilling tool and a bit by a detachable thread, is disclosed. The tool comprises a control device, an actuator and an auxiliary part. The control device detects status of wellbore and controls operations of the actuator when the wellbore leans. The actuator pushes a block out against the well wall to generate a radial force, which pushes against the drill bit to prevent deviation and modify the wellbore trajectory. The auxiliary part transmits the indispensable bit pressure and torque for drilling to assist the control device and the actuator to achieve the function. This disclosure can get automatic deviation correction with only mechanical structures. It is simple and reliable, and unlikely to fail in complicated wells without any manual operation.