In advanced 3D dose verification system for radiotherapy based on scintillation light field imaging, it is necessary to use the point spread function to extract the real scintillation light data of each 2D plane from the overlapping images. The point spread functions of the light field camera at different refocusing positions are measured by knife-edge method combining the digital refocusing of light field and the focussing ranging method. And when the refocusing plane refocused at α=0.6, the function expression of Gaussian defocus for checkerboard calibration plate at α=0.7 was given. The law of light field camera point spread function was studied. All point spread functions could be obtained through function fitting by measuring five necessary values and the workload of point spread function calibration caused by the increase of the number of layers in optical layered imaging was reduced by using this law. Introducing the research results into image processing could obtain the real scintillation data of each 2D refocusing surface through deconvolution operation, which would be helpful for the real-time and accurate measurement of 3D dose.