The unsteady characteristic of the pressure-swirl atomization system was studied experimentally in this paper. It was found that the fluctuations of the spray can be divided into two modes: high and low frequency mode, among which the frequency of the high mode is within the range of 500–1100Hz, and the low mode is 100–400Hz. The low mode depends on the turbulent exchange of momentum between the liquid and the surrounding stagnant gas. While the high mode depends on the comprehensive effect of the turbulent vortex generated by the liquid flowing through the atomizer and the natural frequency of the atomizer. Experiments show that the peak frequency of the low mode increases but the peak frequency of the high mode decreases when the pressure drop of the atomizer increases. When the atomizer outlet area size increases, both the peak frequency of the low mode and the high mode decrease. When the deflection angle of the swirl chamber increases, the peak frequency of the low mode increases, while the peak frequency of the high mode decreases first and then increases, reaching the minimum at 60°, and the value at 45° is basically the same as at 80°.