Based on the mass fraction of 65% deuterated DKDP crystals, the fourth harmonic conversion of 1053 nm laser is realized under the condition of noncritical phase matching (NCPM). The experimentally measured crystal temperature is 29.4 ℃ and the acceptance angle width of the crystals under the NCPM condition is around 55 mrad. Effects of the deuterated amount and the temperature of the DKDP crystals in the process of NCPM on the fourth harmonic conversion are theoretically analyzed. The relationship between the deuterated amount and temperature and the changing of the fourth harmonic conversion efficiency versus the double frequency light intensity are numerically simulated. These results provide a theoretical basis for the application of fourth harmonic laser in the high power laser system.