In this paper,experimental and theoretical studies of the output mode characteristics of an in-phase locked gain waveguide array CO 2 laser are reported.The experimental results of the optical oscillation mode frequency,the far-field intensity distribution and the burnt pattern of the sliced waveguide array laser are obtained.A revised mode expression of the rectangle waveguide,which is suited for this waveguide array CO 2 laser,is proposed.The theoretical simulation results based on the revised mode expression are shown to be in good agreement with the experimental results. In this paper, experimental and theoretical studies of the output mode characteristics of an in-phase locked gain waveguide array CO 2 laser are reported. The experimental results of the optical oscillation mode frequency, the far-field intensity distribution and the burnt pattern of the sliced ​​waveguide array are obtained. A revised mode expression of the rectangle waveguide, which is suited for this waveguide array CO 2 laser, is proposed. the theoretical simulation results based on the revised mode expression are shown in be in good agreement with the experimental results.