As Tonks-Langmuir lower pressure discharge plasma theory is compared with the operating condition of typical Ar~+ lasers,it is considered that this theory may be adaptive for the analysis of plasma parameters in Ar~+ lasers.It is believed that the two-step process plays an essential role in the excitation mechanism of CW Ar~+ lasers.Consequently,the dependenoe of the upper laser level population, which is in approximate proportion to laser output,on the filling-gas pressure in CW Ar~+ lasers is derived.The optimum pressure and electric temperature can then be determined.This result is in agreement with the experiment by H.Beorseh et al.The deviation between theoretical and experimental results in increasing pressure is explained.Some influential factors for determination of the optimum pressure are also discussed.