To study the output mode distribution of the frequency doubling resonant in PPLN, the field distributions and beam quality M2 factor of the plano-concave resonator or dual-cavity were calculated and compared. The output fundamental modes of them with tilted Gaussian-reflectivity mirrors were simulated with Fourier transformation method and Fox-Li iterative method. The results show that the output fundamental mode of the frequency doubling external resonants is Gaussian distribution, and cavity length changes have less effect on the output beam quality. The output beam quality of the plano-concave resonator is M2=1.296, divergence angle θ=58.084 μrad, and the Dual-cavity is M2=6.930, θ= 310.56 μrad. The output beam quality of the plano-concave resonator is superior to the Dual-cavity.