On the basis of Fraunhofer diffraction theory,the diffraction patterns of a Varied Line-space(VLS) grating were obtained for incidence beam in parallel and uniform.The intensities of the diffraction spots changing with diffraction angles were figured out and pictured,when the diameter of incident spots were 5 mm and 10 mm to the same VLS grating,and 10 mm to different VLS gratings.The result was validated by the experimentation for the sensor's resolving power when the diameter of incident spot was different to the same VLS grating.The result reveals that the diameter of incident spot is very important to the sensor's resolving power for the diffraction pattern of VLS grating.