A thermal print head positioning device is disposed in a printing device having a CPU and comprises a thermal print head, a transmission set is made up by a transmission component and a shaft, a cam, an actuator comprising a rotating shaft, a checking plate comprising a plurality of indentations with different sizes, a sensing component, and a supporting frame. The cam and the checking plate are disposed on the shaft. The actuator is electrically connected to the CPU and utilized for actuating the transmission set. The sensing component is disposed on the checking plate and electrically connected to the CPU, and utilized for sensing the sizes of the indentations to make the CPU determine to accelerate or decelerate the actuator. The supporting frame is connected to the cam and the thermal print head, driven by rotation of the cam to make the thermal print head rise or descend.