THz wave has high research value and broad application prospects in many fields. One of the keys for wide applications of THz technology is to develop THz sources with high power, high energy, high efficiency, room temperature operation, tunability, low cost, and portability. An optically pumped gas THz laser has the advantages such as high power, high energy, wide wavelength range, and reliability, which make it be a reliable THz source in the fields of perspective imaging and nondestructive testing. In this paper, the 496 μm THz wave is investigated, which is generated by using the 9P(20) branch line of a Transversely Excited Atmospheric(TEA) CO2 laser when pumping the CH3F gas. In addition, the influences of some factors on the output power of THz are also analyzed. The maximum THz pulse energy obtained in the experiment is 57.14 μJ, and the wavelength is 490 μm.