Optical-path simulator is used to simulate the laser energy density distribution in laboratory.It consists of spherical mirrors.By set the number of spherical mirrors and the distance between them,it can simulate the energy density distribution of the laser spot focused on a practical target a certain distance away.The expressions of the focal length and the back intercept are derived and the computing results are validated by the method of matrix.The results show that they match with each other very well.Some problems of the optical-path simulator in practical application are also discussed.It′s helpful to the using and adjusting of the optical-path simulator.