A grazing incidence diffuser with double-sided triangular prism arrays is researched, which is suitable for edge-lighting light emitting diode (LED) flat panel light without light guide plate. Based on Fresnel equations, the anti-reflection principles of triangular prism arrays are analyzed. The influence of parameters of the triangular prism arrays, i.e. the ratio of depth to width and the bottom width, on the diffuser transmittance is investigated by using the simulation of ray tracing software TracePro. Simulations reveal that the transmittance of entry surface increases with the ratio of depth to width, both for polymethyl methacrylate (PMMA) and polycarbonate (PC). The transmittance achieves the maximum while the ratio of depth to width is about 0.5, and keeps invariant when the ratio increases. The transmittance of exit surface decreases when the ratio of depth to width is close to 0.5, because the light has been total reflected twice in the diffuser. The results show that the transmittance of grazing incident ray can be up to 93% in the optimized diffuser.