The longitudinal discharge-excited Ca+ recombination laser operating at 373.7 nm/370.6 nm was studied for the practical purpose. A simple model of temperature field was presented for the Ca+ recombination laser discharge tube. An analytical expression of the radial temperature distribution in the tube was given. The relation among radial temperature distribution, output power restriction and thermal conductivity of the buffer gas was described. Good agreement exists between the calculated and measured results. This physical analysis can be applied to design Ca+ recombination laser.