A copact superradiant optically pumped D2O gas Tereahertz laser was established, The relatin between laser cavfity length and the intensity of the output Terabertz laser was experimentally studied. By solving the semi-classical theory, the relation was theoretically analysed and numerically calculated. AccoMing to the result of experiment and calculation,the intensity of the output THz laser increased unlinearly with the length of laser cavity;for certain gas pressure,operating temperature and pump power,an optimal length cavity was given.