Based on one-dimensional numerical model for resonant cavity, the dependence of laser spectrum on resonant cavity parameters in quarter-wavelength phase-shifted distributed feedback laser diodes (QPS-DFB-LD) is studied. Comparing theoretical calculation result with measured spectrum, the model parameters of QPS-DFB-LD used in theoretical calculation are given. Simulation results show that the minor deviation of QPS (<10%) cannot affect laser spectrum greatly, but brings a minor blue shift and a slight change on the side-mode-suppression-ration (SMSR). Once the deviation of QPS exists, higher output power will be obtained from the facet closer to QPS region while lower output power will be collected from the other facet. The side of output spectrum SMSR closer to QPS region will be slightly higher. After coating anti-reflective film, residue reflection index of both facets will introduce a drift to the laser mode wavelength within certain range, and will deteriorate the SMSR.