This paper reports the results of the experimental study and chemical composition modeling for a DC argon discharge burning above water cathode in the pressure range of 0.1-1 bar and at discharge current of 40 mA. The gas temperature, reduced electric field strength, and emission intensities of some argon lines were obtained. On the base of these data, the modeling chemical composition of plasma was carried out. At modeling, the combined solution of Boltzmann equation for electrons, equations of vibrational kinetics for ground states of N-2, O-2, H2O and NO molecules, equations of chemical kinetics and plasma conductivity equation was used. The calculations agree well with the measured line intensities. It was shown that discharge burns in diffusion mode, at which the stepwise ionization rate of four lower excited states of argon is equal to the rate of charges diffusion losses.