A laser diode (LD) array side-pumped high-power continuous wave 1338 nm Nd:YAG laser was reported. By analyzing the characteristic of transition and corresponding stimulated-emission cross section in Nd:YAG crystal, according to the coupling condition of energy transition and material-determined wavelength, reasonable coupling parameters of laser rod and cavity mirrors were designed. Therefore, the 1064 nm and 1319 nm lasering was suppressed and 1338 nm laser was successfully gained. Using the high power LD side-pumped module, the output of laser with planar-planar cavity were studied under transitions of the output mirror of 5.3%, 7.4% and 11%. The output of the different cavity lengths were studied. The maximal 1338 nm wavelength output power of larger than 100 W was achieved under 555 W pumping power. The optical efficiency of the laser system is more than 18% , the slope efficiency is about 35% and the M2 of the output beam is 36.