Abstract: The high-quality flat reflector with 2 m aperture can be used for image quality evaluation and performance testing of large-aperture optical instruments. However, by the impact of usage environment, the surface accuracy of large flat reflector cannot maintain long-term stability. Therefore, the surface quality needs to fast calibration on site before using. But the conventional full-caliber or sub-aperture interferometry measurement methods are difficult to meet these needs. Due to the high frequency error of mirror introduced in the manufacturing process being in a steady state, the environmental perturbations introduce only low frequency aberrations. And the selected reconstructed wave-front profile followed with sub-aperture slope scanning is more suitable for surface quality calibration on site of large flat reflector. A double-pentaprism with dual-goniometer synchronize differential measurement method was proposed, which could reduce the random error caused by environmental disturbances in long measurement cycle. Moreover,the optical, mechanical and control system of measurement instrument was designed. And two S-H sensors were proposed to use to instead of traditional goniometer for subapertures slope measurement. Currently validation tests show that the wave-front reconstruction algorithms and instrument angular accuracy meet the demand of accuracy for surface quality, the mutual difference is less than 20 nm (root-mean-square,RMS) with the test result of ZYGO interferometric measurement.