The elliptical windows testing technology has become the biggest block of development. In order to resolve the problem, the present testing technology for the asphere is expressed briefly. For the elliptical windows characterized by large tilt and large eccentricity deep asphere, based on the compensation theory, an improved Offner null testing compensator system for elliptical windows test is designed. The testing system includes compensator lens, field lens and a spherical reference mirror. The principle of Offner′s refractive null lens testing system is that when parallel light from interferometer passes through the compensator lens and the field lens, correction wavefront is produced. After transmission through elliptical window and reflection of spherical reference mirror, light returns back according the original path and interferes with the reference light. The interference results display elliptical window shape deviation. The null lens testing system is implemented and the whole design results are demonstrated. For the tested elliptical window with fineness ratio of 1.0 and aperture of 110 mm, the root-mean-square (RMS) value of the final residual wavefront error is 0.0052λ. Design results provide a certain reference to pratical applications.