On the basis of Maxwell’s equations coupled with the rate equations, the lasing threshold of one-dimensional photonic band edge laser is investigated by finite-difference time-domain method with the introduced effective gain distribution factor (EGDF). The size and position in the photonic crystal of gain mdeium directly influence the output properties of the photonic band edge laser, and the EGDF describes the spatial distribution of gain medium in photonic crystal. The numerical results show that the lasing threshold of an eigenmode depends on the EGDF, different eigenmode has different value of EGDF. Those eigenmodes corresponding to higher value of EGDF have lower threshold. With suitable design of size and position of the gain medium, higher EGDF can be obtained, which directly leads to a lower value of lasing threshold. This gives a new approach to decrease the lasing threshold of the photonic band edge laser.