Based on the theory of semi-classical density matrix, a model of double three-energy-level system for terahertz laser line competition is built. The mathematical expressions of absorption coefficient for pump signal and gain coefficients for terahertz laser signals are deduced, and the competitions between 66 μm and 116 μm spectral lines in CO2-9P (32) optically pumped D2O cavity terahertz laser are calculated numerically by means of iteration. The general rules for laser line competition under different operating conditions (including pump power, operating gas pressure and length of cavity) are summarized.