Abstract: The calculation of the intensity second moment of the uniform plane wave far field diffraction spot have discussed and numerical the variation of the second moment with the different cut percentage of high spatial frequency component has given simulation of calculation. It was difficult to judge the beam quality of the uniform plane wave by the way of M2 factor in principle, but how much high spatial frequency to cut off is prescribed, the process may be executed. This cut of high spatial frequency component will occur in measurement of beam spatial parameters.