In this paper, a terahertz slotted waveguide array antenna is designed based on photonic crystal, which can realize efficient radiation of terahertz waves. The electromagnetic wave is fed from the rectangular waveguide at the bottom of the antenna, coupled to photonic crystal waveguide through photonic crystal cavity, and radiated outward through slots at the top layer of antenna. The simulation results show that the antenna achieves a peak gain of 13.45 dBi at 360 GHz, a half-power beam width of 10.9°, and a side lobe level of −13.9 dB. The antenna based on photonic crystal has the advantages of low profile, low loss, and high radiation efficiency, which can be applied to terahertz wireless communication systems.