The relatively low diffraction efficiency of photorefractive holographic recording, especially that of the two-center holographic recording with 90° geometry, is studied, based on the theory of local diffraction. It shows that the diffraction efficiency in 90° geometry can be comparable with that in transmission geometry only when the amplitude of refractive index gratings is more than 10-4 with the same refractive index change and beam width of 2 mm. According to the effects of fringe vibration caused by environmental perturbation on holographic recording, a concept of effective modulation is proposed. Narrow grating space in 90° recording geometry makes it more sensitive to ambient disturbs and leads to low refractive index change. To obtain high diffraction efficiency in experiments with 90° geometry, an active fringe stabilization system is necessary. Furthermore, in two-center holographic recording, the effects of microcosmic optical parameters on diffraction efficiency are also important.