The parameters of the optically pumped far infrared laser have been calculated. The configuration and oscillation characteristics of the first optically pumped far infrared laser in our country are described. Transmissivity of far infrared optical materials at 496μm, such as fused quartz, germanium, silicon, polyethylene, polytetra-fluoroethylene and all kinds of papers etc have been measured by the methylfluoride far infrared laser and the data are given.