In this article the influence of phase mask characteristic on OFG performance is introduced. The theory analysis and experiments show that the characteristic of the phase mask directly influence the power flatten coefficient and the time delay deviation (TDD) of the chirped OFG, then the TDD will influence dispersion effect of chirped OFG, so the parameters of the phase mask must be stricted. By choosing good quality phase mask and controlling the UV scan exposal course, the high quality OFG is fabricated which has the time delay deviation less than 20 ps and its reflectivity is flatness with ripple coefficient less than 1 dB.