Abstract We report on the investigation of resonant behavior of X-ray cavity in sapphire crystal with the calculation of dynamical theory. At 14.41905 keV, the incident beam will excite a back diffraction(1 3 28) beam. At the same time, the incident beam also excite the other two diffraction beam (2 2 0) and (-1 1 28). We divide the article into three parts. First part we use computer program to build a Edward sphere and make sure the reciprocal lattice points (1 3 28),(2 2 0) and (-1 1 28) will be on the Edward sphere at 14.41905 keV. Then build a new Edward sphere to construct a new four-beam diffraction with (0 0 0),(-1 1 28),(-2 -2 0) and (-3 -1 28). Second part we only consider two-beam diffraction case. We choose (0 0 0) and (1 3 28) to build a Fabry-Perot cavity designed by two parallel sapphire crystals mirrors and make calculation to observe the resonator behavior. Second, different with traditional normal incidence, we use the inclined beam as our incident beam to build a new kind of Fabry-Perot cavity. Because of reversible of light beam root, we use the (-1 1 28) or (2 2 0) diffraction light beam as our incident direction. We calculate and investigate the resonant behavior of the inclined-incidence X-ray cavity. Third, we consider four-beam case. At this time we again make calculation of the normal-incidence X-ray cavity and inclined-incidence X-ray cavity. With the adjustment of crystal thickness and crystal gap, we observe resonant patterns of two different cavities. Finally, we compare the inclined- incidence case to the normal -incidence case, analyzing the intensity and resonant efficiency of two cases.