Defocused spot parameters of an optical system were measured by a testing system with CCD microscopic imaging quantitatively, and an evaluating method for the defocused spot parameters was designed. Firstly, the definitions of the defocused spot parameters including the diameter and the roundness were given. The connected region shaped by the energy contour of the defocused spots was analyzed, and the diameter equal to the area which is 80% of the total energy of the defocused spots was calculated. Then, the boundary of the region was fitted as an ellipse and the roundness of defocused spot was obtained. The method introduces the ellipse fit into the defocused spot roundness measurement through analysis of the energy distribution of star point image on image plane, so that the effects of CCD noise and stray light in test environment on the measuring results are reduced and the confidence of the test results is improved. The experimental results show that the repeatability of the diameter is 0.18 μm and that of the roundness is 1.65%. The method meets the needs of the optical systems in space projects for the imaging quality control. ©, 2015, Chinese Academy of Sciences. All right reserved.