As 2 mm laser is in eye-safe areas and weak atmospheric absorption band, thulium-doped fiber laser attracts a lot of attentions. The lasers output power characteristics of thulium- doped double- clad fiber under laser diode (LD) backward pumping using a plane mirror and concave mirror are compared. Experiments and theories suggest that the laser cavity losses increase because the gap, tilt, and fiber end face defects between the fiber end face and the plane mirror and mirror thermal damage and other factors, and the laser output performance is severely affected. The cavity insertion loss properties are analyzed after using a plane mirror and concave mirror respectively at the end of the fiber based on the wave theory, which suggests that using a concave mirror cavity the loss is smaller than that using a plane mirror. When using the concave mirror cavity structure after fiber end, the maximum output power of 22 W is obtained, corresponding to the center wavelength of 1998.6 nm and the slope efficiency of 43% relative to the incident pump power. Comparing to the use of plane mirror cavity the laser slope efficiency increases of 10% , and the thermal damage of mirror is alleviated obviously.