Abstract: 【Objective】The pathogen causing leaf spot disease on melon in greenhouse in Gansu in recent years was clarified to provide reference for the effective control of the disease.【Method】In this experiment, the pathogens of melon leaf spot disease, their Biological characteristics and fungicides screening in laboratory were studied by tissue separation method ,single spore isolation method and growth rate method. The pathogens of melon leaf spot disease were identified as Alternaira peponicola (Rabenhorst simmous) and Alternaria cucumerina (Ell. et Ev. Elliott.).The optimal temperature for the growth of these two Alternaria were 30 °C,the light conditions and the suitable medium were completely light and completely dark, melon juice and PDA of Alternaira peponicola (Rabenhorst simmous) and Alternaria cucumerina (Ell. et Ev. Elliott.), respectively. The indoor fungicides screening found that the different fungicides had different inhibition rates for different pathogens, the inhibition rate of 10% difenoconazole, 40% flusilazole, 50% thiram, 70% mancozeb against Alternaira peponicola (Rabenhorst simmous) were above 84%, which were 96.66%, 94.46%, 90.82% and 84.37%, their value of EC₅₀ were 17.89μg/mL, 3.41μg/mL, 44.27μg/mL, 42.34μg/mL, respectively. 50% ether oxystrobin, 43% tebuconazole and 40% myclobutanil had better inhibition against Alternaria cucumerina (Ell. et Ev. Elliott.),the inhibition rates were all above 85%, 98.20%, 90.82% and 85.43%, respectively, and their EC₅₀ were 3.45 μg/mL, 21.35 μg/mL, and 5.21 μg/mL, respectively. 【Conclusion】These results provide a basis for further development and application of the above fungicides. The results provide a basis for effective control of melon leaf spot disease.