The output characteristics of LD end-pumped single-longitudinal-mode (SLM) 3 μm lasers are studied theoretically.Based on the analysis of energy processes of the Er:YAG laser system,a rate equation model with the influences of thermal effect taken into account is established,and the effects of the temperature and doping concentration and length of the laser crystal,the pump waist to laser waist ratio,the transmission of output coupler,the insertion loss of the optical diode on the output behaviors of the continuous wave (CW) SLM 3 μm laser are researched.The model can be used to guide the design and optimization of a high power CW SLM Er:YAG laser operating at 3 μm.