Abstract: The fragmentation of melt droplet is a key factor to predict the damage degree of fuel coolant interaction accident, which determines the final ratio of internal energy transferred to power. However, the fragmentation mechanism based on internal energy is not very clear currently. The interaction of single melt droplet with water was studied by experiment in the paper. Thermal fragmentation of melt droplet was observed with high-speed camera. Experiment results show that the interaction goes through several circles of expansion. The measured fragmentation time is about 0.8 ms, the interval time of two fragmentations is 0.8 ms, and the accelerating expansion time is 0.4 ms. A new thermal fragmentation mechanism based on boiling effect was proposed.