Abstract: The paper studies the cooling system of a diode laser stack, and presents a phase transition cooling system using R134a as coolant and a heat sink based on throttle microchannel phase transition cooling. The packaging of the quasi-continuous wave (pulsed) 3 kW laser diode stack is finished. The influence of the temperature difference of coolant at inlet and outlet on the output wavelength of the stack is analyzed. Experimental results showthat the stack achieves a 3 030 W peak light-output power with a duty cycle of 20% at a current of 197 A. The wall plug efficiency is 39% and the vaporization rate of R134a in the cooler is 50%. The half width at full maximum of spectrum is smaller than 3.8 nm. The flow rate of the coolant R134a is 0.60 L/min, which is about 20% that of water. Therefore, the coolant flow becomes smaller and the volume of the thermal management unit diminishes significantly.