This cubic wavefront coding phase plate is an asymmetrical surface.Up to now,its main testing method in the industry is using the contact profiler.But non-contact testing method has more virtues,for example,simple,swift,unharmful to the surface under test.Non-contact testing method can use computer-generated hologram（CGH） as null lens,but the CGH manufacture cost is comparatively expensive.We gave a new manner of null lens to test the cubic surface according to the surface characteristic of the cubic wavefront coding phase plate.We described a design of the null lens and made some experiments.The null lens consists of two spherical lenses which are easy to be manufactured at a low price.