Testing of the large-aperture convex aspheric mirror is a very difficult task in the aspherical testing. The null lens compensator is one of the efficient methods for testing the large-aperture convex aspheric mirrors. However, with the aperture increasing, some unavoidable errors are induced in the fabrication and adjustment of the compensator. In this paper, a method using a standard sphere mirror is proposed to calibrate the error of the compensator. The system error in the compensator is tested and calibrated. The calibrated compensator is used to measure the large-aperture aspheric mirror. The calibration error is also analyzed. The root-mean-square (RMS) error before calibration is 0.097 λ (wavelength λ=632.8 nm), which significantly deteriorates the testing accuracy in the null lens testing. The final testing RMS error after calibration is smaller than 0.0046 λ, which satisfies the designed precision requirement of the large-aperture convex aspheric mirror.