It is difficult to fabricate and test convex aspherical surfaces, especially for steep convex aspheical surfaces. Based on the analysis of the conventional null test, a simple test system with cemented doublet for two steep convex ellipsoids was discussed. The wavefront of the both test system was less than 1/30λ RMS (Root Meat Square, λ=632.8 nm). The convex aspheric lens had their successful application in practical optical system. The result showed that the null test for convex aspheric lens discussed in this paper is valid. Compared with the traditional test method, the cemented doublet offsets the spherical aberration generated by the sphere surface of the lens. So the proposed test method decreases the amount of the spherical aberration generated by the aspherical lens, with fewer elements, and reduces the system′s alignment difficulty. It was demonstrated to be an effective, high-precision testing method.