Abstract: This study explored the toxicity of the combined application of fungicide procymidone and fludioxonil on cucumber gray mold, and then analyzed their residual dynamics, which will be helpful to enhance the risk prevention and control levels of two fungicides for the cucumber and its habitat environment. The EC₅₀ of procymidone and fludioxonil against Botrytis cinerea were determined by the mycelial growth inhibition method and were 0.069 mg/L and 0.103 mg/L, respectively. The EC₅₀ value of the mixture of procymidone and fludioxonil at a 1﹕1 ratio against B. cinerea was 0.016 mg/L and the synergistic ratio was 5.0, indicating a strong synergistic effect. The results of scanning electron microscope showed that the mycelia of B. cinerea showed mycelia shriveled and intracellular material overflowed, respectively, when treated with procymidone and fludioxonil separately for 3 days; Whereas, the mycelia of B. cinerea showed mycelia shriveled and intracellular material overflowed at the same time when treated with the mixture of procymidone and fludioxonil at 1﹕1 ratio for 3 days. These results confirmed the duplicate effect of the two fungicides. To further analyze the dynamic of pesticide residue after using the mixture of procymidone and fludioxonil, a high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) method was established, which could detect procymidone and fludioxonil concurrently. The recovery ratios of procymidone and fludioxonil were 100%-102% and 94%-96%, respectively, the RSD of procymidone and fludioxonil were 1.7%-5.3% and 2.8%-3.4%, and the LOQ was 0.01 mg/L, which met the testing requirements. The field experiment showed that the final residue was reduced by 59% and 86% when using the mixture of procymidone and fludioxonil at 1﹕1 ratio compared with using the two fungicides alone. The half-life was 5.38 d and 6.93 d respectively for procymidone and fludioxonil when used alone, and the half-life was reduced to 4.39 d and 4.33 d when the mixture of the two fungicides was used. These results suggested that the mixture of procymidone and fludioxonil has an increased toxicological effect against B. cinerea. The conclusion provided a theoretical basis for reducing fungicides and was meaningful for reducing the pesticide residue risk on cucumber.