A model for magnetically confined discharge carbon monoxide laser is presented. Electron energy distribution function, the influence of the magnetic field on electron impact excitation, the vibration-to-translation and vibration-to-vibratio n excitation rates of CO molecule are discussed. The vibrational-state populatio ns of CO molecule and small-signal gain with magnetic field are calculated and s tudied also. After a magnetic field has been added， there appears a hole in the distribution of the vibrational-state populations of CO molecule and the small- signal gain is higher in this region.