To detect the size distribution of compound fertilizers accurately, an overall calibration method was proposed for the CCD based visual detective system of a particle size analyzer. Firstly, the detection accuracy of the inspective system was analyzed and the light source was controlled in servo to reduce the effect of instability and asymmetry of the light source on the detection accuracy of the visual detective system, Then, an appropriate scanning speed of CCD was determined to insure the real shape of the object to be detected. Finally, based on the theory of the ruled surface, the pixel diameters and pixel coordinates were obtained through measuring the balls with different diameters for several times. The overall calibration of the whole detective system was implemented by using Excel, MIL and other tools to establish mathematical model of ruled surfaces. Experiment results show that the non-uniformity response of the CCD vision inspection system has been significantly improved, and the detection error is within ± 3%. It is simple, intuitive, feasible, and can meet accuracy demand of the particle size analyzer.