An evaporator, applicable to a two-stage expansion cooling system, is used for receiving a high-pressure liquid working fluid. The evaporator includes a thermal-conductive block having a channel system. The channel system includes a high-pressure channel, a low-pressure channel, and a second stage expansion channel. The second stage expansion channel has an input end and an output end. The input end is communicated with the high-pressure channel. The output end is communicated with the low-pressure channel, and has a cross-sectional area smaller than that of the low-pressure channel. The high-pressure liquid working fluid flows into the thermal-conductive block from the high-pressure channel, and then enters the second stage expansion channel through the input end. A part of the high-pressure liquid working fluid flowing out of the output end expands into a saturated low-pressure liquid working fluid and enters the low-pressure channel.