In this paper the effective index perturbation technique in combination with two-dimensional (2D)/three-dimensional (3D) plane wave expansion methods is used to predict resonant mode frequencies of donor-like and acceptor-like H1 photonic crystal slab cavity, and their results are very close to the ones obtained by three-dimensional finite difference time domain method. For donor-like H1 photonic cavity, when the perturbed effective index by matching dielectric band edge is used, there is a relatively small frequency difference; however, for acceptor-like H1 photonic cavity, the matching criterion should be set at middle band position. The effective index perturbation method can not only save computation time and computer memory with reducing dimensionality (from 3D to 2D), but also ensure the accuracy of the simulation results, which is substantially important for the extensive application of photonic crystal slab cavity.