Using the least-squares spline approximation to analyze the photonic band structure, the dispersion parameters of a two-dimensional photonic crystal are quantitatively calculated. Considering the large bandwidth of an ultrashort pulse, average group velocity and average group velocity dispersion are achieved by introducing a weighting factor, which reveals the share of different frequency components. So the pulse evolution can be predicted theoretically. Propagation of ultrashort pulses through the photonic crystals is simulated by using the finite-difference time-domain method. The results of numerical simulations are well consistent with the theoretical predictions. This process for achieving dispersion parameters is valuable for designing photonic-crystal dispersion devices such as dispersion compensator, pulse compressor and pulse stretcher.