A set of cryogenic measurement system for semiconductor lasers with microchannel structure was developed. The stable measurement of some vital parameters such as output power, electrooptic conversion efficiency and spectra of high power semiconductor lasers in the range from －60℃ to 0℃ were realized. Based on computational fluid dynamics and numerical heat transfer methods, the heat dissipation performance of three cryogenic coolants, anhydrous ethanol, trichloroethylene and pentafluoropropane was simulated. The simulation results show that semiconductor laser bar with anhydrous ethanol as the coolant has the smallest thermal resistance(0.73 K/W) and the best temperature uniformity(temperature difference between emitters is 1.45℃) when the pressure drop is 0.47 bar. Anhydrous ethanol was used as system coolant, the maximum of ethanol flow rate was up to 0.5 L/min, and 5 semiconductor laser bars could work simultaneously in the system. Based on the cryogenic measurement system, the cryogenic characteristics of 976 nm semiconductor laser bar with microchannel structure at 6% duty cycle were investigated. The experimental results show that the output power of semiconductor laser bar is increased from 388.37 W to 458.37 W which the powerup ratio is 18.02%, the electrooptic conversion efficiency is increased from 60.99% to 67.25%, the efficiency is increased by 6.26%, and the central wavelength is shifted from 969.68 nm to 954.05 nm when the coolant temperature decreases from 0℃ to －60℃. The turnon voltage increases by 0.04 V, the threshold current decreases by 3.93 A, the series resistance increases by 0.18 mΩ, and the external differential efficiency increases by 11.84%. The analysis shows that the decrease of threshold current and the improvement of external differential efficiency are the main factors that promote the power and efficiency of semiconductor lasers at low temperature. This investigation shows that the cryogenic working mode of liquid microchannel cooling is an effective means to achieve high output power and high electrooptic conversion efficiency of semiconductor lasers.