The Mach-Zehnder interferometer and phase-diffusion main equation were used to simulate the influence of atmospheric phase fluctuation on the quantum radar, and the evolution process under phase diffusion channel was solved. The influence of atmospheric phase fluctuation on the phase estimation resolution and optimal sensitivity of the coherent state parity probe quantum radar were thoroughly analyzed, and the Cramer-Rao limit of phase estimation sensitivity under atmospheric phase fluctuation was calculated. The study found that the effect of phase fluctuation on resolution can be eliminated by increasing the average number of photons per pulse. Moreover, in the case of strong phase fluctuation, the best sensitivity of parity detection may seriously deviate from the shot noise limit, but in the case of weak phase fluctuation, it is found that the parity detection is a quasi-optimal detection compared with the Cramer-Rao limit.