The fragmentation process of high-temperature molten drop is a key factor to determine the ratio heat transferred to power in FCIs, which estimates the possible damage degree during the hypothetical severe accident in the nuclear reactors. In this paper, the fragmentation process of melt droplet in FCIs is investigated by theoretic analysis. The fragmentation mechanism is studied when an external pressure pulse applied to a melt droplet, which is surrounded by vapor film. The vapor film collapse which induces fragmentation of melt droplet is analyzed and modeled. And then the generated pressure is calculated. The vapor film collapse model is introduced to fragmentation correlation, and the predicted fragment size is calculated and compared with experimental data. The result shows that the developed model can predict the diameter of fragments and can be used to calculate the fragmentation process appreciatively.