Abstract: Objective: To investigate the changes of cell wall degrading enzyme activity and gene expression during the infection of postharvest melon fruit by Alternaria alternata, and to reveal the pathogenic mechanism of Alternaria alternata. Methods: Jiashi melon and 86-1 melon fruits were used as experimental materials. Alternaria alternata was inoculated and stored at room temperature (25 ℃) and relative humidity of 35%~40%. The diameter of lesions was measured, and the microstructure of lesions was observed. Pectin lyase (PL), cellulase (Cx), β-glucosidase (CB) and polygalacturonase (PG) were determined. The expression changes of AaPL, AaCx, AaPG and AaCB were analyzed by absolute quantitative method of fluorescence quantitative polymerase chain reaction (PCR), and the correlation between the activity of four cell wall degrading enzymes and the expression of their corresponding genes was analyzed. Results: Under the same infection days, the lesion diameter of 86-1 melon pulp and peel tissue was larger than that of Jiashi melon. The microstructure of the lesion tissue was observed. Compared with the two melons, A. alternata propagated faster in 86-1 melon cells and had a stronger ability to destroy the cell structure of 86-1 melon. The changes of activity and gene expression of four cell wall degrading enzymes on two melons were compared. Under the same number of days, the enzyme activity and gene expression were higher in the 86-1 melon inoculation group, and the number of days when the activity and expression reached the peak in the Jiashi melon inoculation group was later. Conclusion: During the infection process, due to the melon's own disease resistance mechanism, the change trend of cell wall degrading enzyme activity and gene expression fluctuated from 6 to 10 days. The infection ability of Alternaria alternata in 86-1 melon was stronger, which also reflected that Jiashi melon had stronger resistance to Alternaria alternata infection.