The structure of a simultaneous phase-shifting interferometry system based on two-dimensional (2D) orthogonal grating diffraction is analyzed in detail, including components of interference, spatial beam splitting, polarizing phase-shifting element, and image sampling and processing. The four diffraction-order beams of the 2D orthogonal transmission grating are taken as the testing beams. Both the theory and the experiment show that the beam splitting of the system function well. A polarizing array, consists of four linear polarizing elements of which transmitting directions change 45° in turn, acts as the phase-shifter. According to the requirement of spatial consistency, the four independent interferograms with 90° phase shift are divided. High phase-shifting precision is obtained. The repetitive experimental results show that, when the amplitude-frequency product of environmental vibration does not exceed 100 Hz λ, the peak to valley of testing repeatability is less than 0.02λ, so the advanced system can be used in on-line testing and dynamic measurement.