The model-based wavefront sensorless adaptive optics (AO) system has a great potential in real-time correction because of its fast convergence and good correction. The linear relationship between imaging masked detector signal SMDand mean square gradient of wavefront phase SMG is the theoretical basis of the system control algorithm. To verify the linear relationship, we establish an AO system experiment platform with a 37-element piezoelectric deformable mirror(DM), a CCD camera and a Shack Hartmann sensor, in which the DM is used to generate wavefront aberration, the CCD camera is used to grab the distribution of far-field intensity and the Hartmann is used to get the information of wavefront. Based on the collected information of far-field intensity and the wavefront aberration, we calculate values of SMD and SMG, respectively. Experiment results show that SMD is linear to SMG and the slop is 0.018, which is very close to the theoretical value 1/(4π 2).