In order to understand the process of phase graing imaging intuitively and deeply, a new analysis method was proposed, which is called as the Principal Diffraction Orders Analysis (PDOA). The key point of PDOA for analysing the imaging of the phase grating is focusing on the principal diffraction orders of phase grating, while ignoring the non-essential diffraction orders, which will greatly simplify the analysis of the phase grating imaging. The analysis result of PDOA indicates that for a π-phase shift grating with 50% duty cycle, the principal diffraction orders are ±1 diffration orders, but ±3 diffraction orders are also necessary. And for a π/2-phase shift grating with 50% duty cycle, the zeroth diffraction order is not merely creating annoying background radiation. It is as important as ±1 diffration orders of phase grating.