In order to ensure the imaging quality and modulate the focusing plane, a focusing mechanism based on a constant-diameter conjugate cam is designed. To improve focusing accuracy, factors affecting the accuracy of cam are studied. The influences of the roller radial runout, cam machining error and the base circle radius on focusing accuracy are analyzed. Then, a method to guarantee the cam diameter constant and remove return stroke error is studied by using least-squares method. The analysis results indicate that the focusing accuracy can reach ± 4.7 μm, and the return stroke error can reach 4.8 μm. The functioning of institutions is stable and reliable.