Abstract: In order to guide the scientific preparation of cotton harvest aid solutions, the changes in physicochemical properties of four harvest aid solutions with different storage times and their effects on defoliation efficiency sprayed by plant protection unmanned aerial vehicles (UAV) were evaluated. The results showed that the suspension rate and retention of 540 g/L thidiazuron + diuron SC system at 0 h were the best, which were 95.5% and 5.49 mg/cm², respectively. 68% Thidiazuron + diuron WP solution system had the best contact angle on cotton leaves at 0 h (0°). The stability of thidiazuron in 81% thidiazuron + diuron WG and 12% thidiazuron + diuron OD was acceptable, and the decomposition rates decreased by 11.63% and 14.47% respectively after 24 h. In general, the suspension rate, retention, and thidiazuron content of the four harvest aid solutions decreased with the extension of storage time, and the decrease was the most significant at 0.5-1 h. The storage time of the solution has different effects on the droplet volume median diameter(VMD, D_v50), droplet coverage, and droplet uniformity when sprayed by the plant protection UAV. The defoliation rates of the four kinds of cotton harvest aid solutions had little variation at each storage time and decreased with the extension of storage time. The defoliation rate decreased the fastest within 0-1 h. The four harvest aid solutions had a significant effect on the defoliation rate of cotton after being placed for more than 1 h, and the effect was the greatest after being placed for 12 h after preparation. These results can provide a theoretical basis for the scientific preparation and spraying of cotton harvest aid solutions.