The pepper root system and soil samples were collected from the pepper planting area in Qimaba Township, Lvchun County, Honghe Prefecture, Yunnan Province. To investigate pepper phytophthora root rot and calculate its incidence and disease index, The spore density, richness, diversity index and mycorrhizal infection rate of arbuscular mycorrhizal fungi（AMF） in the rhizosphere and around roots of pepper were investigated by wet-screen precipitation method and Plant root staining. Then the final data were analyzed by SPSS software. The results showed that when the mycorrhizal infection rate was 62.5%, the spore density was also the highest, reaching 11.725 spores/ g, the incidence and disease index of pepper phytophthora root rot were the lowest (20% and 9, respectively). when mycorrhizal infection rate and spore density were the lowest 11% and 2.475 spores/ g, the incidence and disease index of pepper phytophthora root rot were the highest at 80% and 53.5, respectively. The results of the correlation analysis showed that the spore density, richness and mycorrhizal infection rate of the AMF were negatively correlated with the incidence and the disease index of pepper phytophthora root rot. That is, the higher the spore richness and spore density of AMF, the higher the mycorrhiza infection rate, the lower the incidence of pepper phytophthora root rot and the smaller the disease index. It follows that the existence of AMF in pepper root soil can enhance the disease resistance of pepper and reduce the occurrence of diseases.