The basis of two algorithms-transfer matrix method and block-iterative frequency-domain method are introduced to calculate two-dimensional (2D) photonic crystal. The computational domain is a 2D structure of square lattice that contains 5×5 dielectric rods in air. By changing the size of the radius of the central rod, we get their transmission and dispersion relationship. To the point-defect 2D photonic crystals, the two kinds of curves consist of the same photonic band gaps and defect states and embody the characteristics of photonic crystal from different aspects. Because of the difference of the central rod radius, different band gap range and different defect state frequencies are acquired. In our article, according to the transmission and dispersion curves of the different parameters, we analyze the practicability of using point-defect photonic crystal as laser resonant cavity. It is found the defect structure has the function of canceling the lowest-order mode by changing central dielectric rod size and dielectric constant,independent of the other modes .A mode fraction of radiated power is concentrated in the lowest-order mode ,canceling the lowest-order mode can reduce energy radiation and lead to improve quality factor.