With the rapid development of modern space technologies, heat sinks with high heat flux have become a bottleneck in the development of high capacity electronic equipment with high power densities. To ensure the stability and security of heat sink devices, it's crucial to greatly enhance the heat transfer performance of the heat sink device and flow boiling in microchannels has attracted more and more attention due to its unique advantages. The objective of this investigation is to study the growth and heat transfer of single bubble and multiple bubbles during flow boiling in a microchannel using the VOSET method. In order to verify the accuracy of the numerical model adopted in this paper, the results obtained by the present method were compared with that of previous experiments and numerical calculations. After that, effects of Re and wall superheat on the bubble morphologies and heat transfer in a microchannel were studied. Increasing Reynolds number could increase the bubble growth rate and boiling flow heat transfer. However, there was a slight increase in heat transfer performance with an increase in wall superheat. In addition, when Re reached a certain value, the elongated bubble zone would appear in the microchannel. Further, the stretching length, the contact length between the heated wall and the vapour and the second kind of thin liquid layer were analyzed in detail. Finally, more importantly, the dynamics and heat transfer of multiple bubbles with different bubble waiting time were presented. With a decrease in bubble waiting time, the heat transfer performance was enhanced significantly.