[Background] Powdery mildew is one of the main fungal disease affecting the yield and quality of alfalfa. It has been reported that there are three pathogens causing alfalfa powdery mildew: Erysiphe polygoni DC., Leveillula leguminosarum Golov., and Erysiphe pisi DC. [Objective] The biological characteristics of alfalfa powdery mildew, the morphological characteristics of conidia and cleistothecium were systematically studied, and the pathogenic species of alfalfa powdery mildew in Ningxia were identified. [Methods] The occurrences of the disease and the formation conditions of the cleistothecia were investigated by pot culture in the laboratory. The data were obtained automatically by the meteorological station. The characteristics of the fungi were observed by the light microscope, and the germination characteristics of the conidia of the fungi were observed by the water agar slide method. [Results] The temperature range of alfalfa powdery mildew is 20?24 °C, the humidity is 40%?60%. The conditions for the formation of cleistothecia are 20?25 °C and humidity about 70%. The formation of cleistothecium takes about 42 d. Conidia produce spores on conidiophore in a basipetal form. Mature conidia oval or elliptic, with the size is (29?46)×(12.5?21.0) μm. The diameter of dark brown globular or subglobular cleistothecium is 71?135 μm, with a large number of appendages, and a few are irregular branches 1?2 times. The cleistothecium contains 4?6 asci, oval, with the handle, and the size is (58?90)×(29?60) μm. The ascus contains 4?6 ascospores, which are ascospore monospora, oval and the size is (21?34)×(10.5?20.0) μm. The optimum temperature range of conidia germination is 22?26 °C, the optimum pH is 6.5, and the light could promote conidia germination. Glucose, D-xylose and mannitol promoted the germination of conidia and the growth of bud tube, cysteine promoted conidia germination, and glycine promoted the growth of bud tube. [Conclusion] The pathogen of alfalfa powdery mildew in Ningxia was identified as Erysiphe polygoni DC.