In this article, by design of a single lens, perfect optical null compensator, flat output wavefront of an interferometer is converted to parabolic wavefront and by that, parabolic surfaces can be tested. In method that is presented in this article, design of null compensator is done by using of one single lens that one side of that is aspherical. By using this single element null compensator, in comparing by two elements null compensators, it is not necessary to do accurate adjustment of distance between these two lenses and also it is not necessary to do accurate adjustment of alignment of them and according to these, accuracy of measurements is increased significantly. In addition, by reduction of number of lenses, surface error of lenses and also error of homogeneity of optical materials is reduced significantly.