Phase-shifting interferometry (PSI) is widely used in optical testing because of its advantages of high accuracy, high sensitivity and untouched test characteristics. However, the environment vibration introduces a random error in the temporal phase shift, which should be presise enough to ensure the test accuracy. In this paper, an novel simultaneous phase-shifting interferometry (SPSI) device is presented. In this device, using two identical transmission gratings and double shear-plates as the beam splitter and double shear-plates as the phase shifter, the optical path difference between the original wavefront and the sheared wavefront can be achieved by adjusting the spacing and wedge angle between the double shear-plates. Then three interferograms with 90° phase shift interval are captured simultaneously and instantaneously by the same CCD. The processing of the interferograms is performed using conventional methods of phase extraction with the algorithm of three phase shifts. Overcoming the disadvantages of traditional mechanical phaseshifting mode, therefore, influence caused by the environment vibration is eliminated. The arithmetic model of this SPSI system is deduced, sheared interferograms of optical system with different wavefront heights can be obtained, which can be used to test the high precision optical surface on-line. Compared with the conventional temporal PSI, this SPSI device is easy to be realized, but some phase shift errors appears such as the spatial mismatch between phase shifted interferograms and the inaccurate light split ratio generated by the difference diffraction efficiency between the ±1th order and the 0th order transmittance lights of the gratings. Better precision can be achieved by error correction techniques such as phase shifting calibration or multiplying the right correction factor in the interferograms processing algorithm.