Abstract: In view of the lack of prediction model of droplet size of the liquid jet in a coaxial airflow, combined with the linear stability theory of the liquid jet, the mathematical expression of droplet size of the liquid jet in a coaxial airflow based on the critical modulus is established in this paper. On this basis, the effects of surrounding gas twisting (the surrounding airflow has both axial and circumferential motion) and fluid physical parameters (gas compressibility, liquid viscosity, gas liquid density ratio, and surface tension) on droplet size of liquid let are studied respectively. The research results show that: (1) both the axial ejection and coaxial rotation of the surrounding airflow will lead to the droplet size increasing first and then decreasing. When there is only rotation motion of the surrounding airflow, the surrounding airflow rotation has little effect on the droplet size of liquid jet under the same critical modulus. (2) Within the range of parameters studied in this paper, the droplet size of liquid jet decreases with the increasing of the surrounding gas compressibility and the gas liquid density ratio, and the droplet size of liquid jet increases with the increasing of the liquid viscosity and surface tension. The effect of gas compressibility on droplet size of liquid jet is stronger when the surrounding airflow rotates coaxially, while the effect of liquid viscosity on droplet size of liquid jet is more significant when the surrounding airflow ejects coaxially. The research results have certain theoretical significance and engineering application value for the droplet size prediction of liquid jet in a coaxial airflow.