Aiming at the instability of boiling flow in parallel microchannels heat sink, the boiling flow and heat transfer characteristics of microchannel heat sink with flow sharing cavity with inner arc transition (MC-C) and microchannel heat sink with traditional square flow sharing cavity (MC-S) were studied. Using R134a as refrigerant, the two-phase flow pattern, wall temperature and heat transfer coefficient of microchannel were analyzed under the conditions of mass flow rate of 416~728 kg/(m2?s) and heat flux of 36.7~242.6 kW/m2. The working medium flowed into the inlet flow sharing cavity through the inlet pipe, then flowed through the microchannels, entered the outlet flow sharing cavity, and flowed out of the heat sink through the outlet pipe. The results showed that bubble flow, bubble-slug flow, slug flow, and annular flow changed in the channels when the heat flux increased from low to high. Compared with MC-S microchannels heat sink, the inlet flow sharing cavity of MC-C microchannels heat sink reduced the flow resistance of working fluid, and the outlet flow sharing cavity promoted the steam to be discharged from the microchannels heat sink, and the flow pattern in each microchannel of MC-C microchannels heat sink was more uniform. The wall temperature of MC-C microchannels heat sink increased first, then decreased and then increased, while that of MC-S microchannels heat sink decreased first and then increased. Under the same working conditions, MC-C microchannels heat sink can achieve lower wall temperature. The heat transfer coefficient in two kinds of microchannels heat sink increased with the increase of mass flow rate and heat flux. Under the same working condition, MC-C can achieve higher heat transfer coefficient. When the heat flux was 242.6 kW/m2, the wall temperature of MC-C microchannels was 2.8℃ lower than that of MC-S microchannels, and when the mass flux was 572 kg/(m2?s), the maximum temperature difference of MC-C microchannels was 2.2℃ lower than that of MC-S microchannels. When the heat flux was 242.6 kW/m2, the average heat transfer coefficient of MC-C microchannels was 20.2% higher than that of MC-S microchannels.