This paper describes a method of "optical tracing" used to measure the energy distribution on focal plane of an imperfect processed optical system. It is similar to Hartmann method. High brightness He-Ne laser with TEM00 mode output is used as a probing beam. A parallel beam passes through a movable aperture and is focused by a local part of optical system to be measured. Then we determine the position of central point of Airy spot on the focal plane. The spatial coordinate of the point is magnified to 40X by an objective and read out by microscope. Coordinate of each central point of Airy spot corresponding to each local part is obtained if the aperture can be moved in the whole optical system. In the case that the optical system is divided into m parts by the movable aperture, there will be m points on focal plane. The ratio of the point numbers on a local part of the focal plane to to is proportional to the relative energy of incident beam focused on it. The accuracy of this method can be up to the magnitude of Airy spot diameter of the whole optical system. The movable aperture is defined as 1/10 of that of the optical system, since the accuracy determining central point of Airy spot of the movable aperture can be up to 1/10 of the spot diameter. The focal spot size and energy distribution on focal plane of aspherical-gpherical lenses for laser-plasma interaction research have been measured using this mathod. It provides basis for improving lens processing and calculating the energy density on target plane.