In recent years, Yttria-stabilized zirconia based thermal barrier coatings (TBCs) are deposited by newly-developed high-efficiency supersonic atmospheric plasma spraying (SAPS) technology. The final microstructure of the plasma-sprayed coatings is strongly dependent on the size distribution of spray particles. It has been corroborated through experiments that there is a special phenomenon of particle melting refining in SAPS, as compared with the conventional atmospheric plasma spraying (APS). This phenomenon greatly affects the final particle size and distribution, which has not been explained reasonably up to now. Therefore, it is necessary to investigate the melting refining behavior of in-flight particles to control the particle size and to analyze the coating properties. In this paper, the breakup of particle is presented to characterize the phenomenon of particle melting refining, and the peak of size distribution becomes bigger with increasing the spray distances, which is explained by collision-coalescence. Furthermore, based on the maximum entropy formalism, the particle-size distribution is calculated and the result is in good accordance with the plasma spraying experiment results, which verifies the mechanism analysis presented in this paper. This work could provide more efficient applications of the SAPS technology in high-performance TBCs.