Based on the solution of the self-consistent problem, the influence of laser mixture relaxing beyond the discharge region upon the continuous 600 W carbon-dioxide laser radiation has been investigated. It has been shown that beyond the discharge region, the resonance nonlinear absorption of radiation can originate in the discharge-free area within the certain range of gas velocity, energy input and resonator Q-factor values. The absorption dependence on radiation intensity is complicated in its nature and involves the influence of saturation effect, as well as the variation of excited molecules relaxation length in the radiation field. The values of output mirror reflection factor and the geometry parameters have been determined whereby the laser efficiency is at its maximum with regard to extra amplification of radiation in the relaxing gas mixture flow.