To reduce the grating groove errors caused by the inclination of a grating ruling platform in the fabrication of diffractive gratings this paper proposes a real-time leveling device for the grating ruling platform based on image clarity measurement and electrically adjusting a tilting table. A physical model was built based on the inclination relations between the image clarity measuring and the inclination angle of grating ruling platform. Based on the model, the leveling device could control the grating ruling platform to implement the closed loop dynamic leveling within a certain range. The above theory was verified by tests. The results show that the leveling device is simple and feasible. For a grating blank with a size of 50 mm×50 mm, it controls the inclination of the grating ruling platform to be within 4″, meeting the requirements of the grating ruling for platform positioning accuracy. Moreover, the leveling device can be used in leveling before ruling and also in ruling processing in real time.