To meet the current demand of climate change for solar spectral radiation observation, a wavelength scanning solar spectral irradiance meter is developed in order to accurately monitor the solar spectral irradiance. The solar spectrum irradiance meter is mainly composed of a spectrum measurement unit and a wavelength scanning unit. As the precise positioning mechanism of dispersion spectrum, the wavelength scanning unit is the key to ensure the measurement accuracy of the instrument. Therefore, a wavelength precision scanning positioning device based on a small AC servo motor is designed. In the device, the rotation of the motor is controlled by a timer, the linear CCD is driven by STM32 to work, then the closed-loop control is realized with the precisionless than 2 pixels based on the deviation between the centroid position detected by the CCD and the preset position. Finally, the repeatability of the linear CCD centroid position detection and the stability of the scanning positioning device are tested. It is shown that the standard deviation of CCD centroid position detection is 0.0693, the maximum deviation does not exceed 0.3 pixels width, and the closed-loop accuracy of the system is less than 2 pixels width during operation, which meets the repeatability and stability requirements of wavelength scanning.