To study the effect of output power on the pressure of cavity in continuous wave (CW) DF chemical lasers, a series of experiments are conducted based on a small-scale combustion-driven CW DF chemical laser. The result shows that greater specific power will result in lower cavity pressure. An algebraic model from combustor to the exit of cavity is constructed by using one-dimension steady state theory and cavity dynamics theory, then the dependence of cavity pressure on specific power is attained. The result is consistent with experiments, so the model explains the effect of output power on the pressure of cavity, and the result indicates that the model is logical. Both the model and experiments show that cavity pressure falls down because of output power. When specific power is 50 J/g, cavity pressure will be reduced by 7.4% theoretically in the experimental condition.