[Objective] Identification of a symbiotic fungus of bag-cultured Auricularia auricula and preliminary studies on its symbiotic effect. [Methods] After purified the symbiotic fungus strain, it was identified by morphology and molecular biological analysis. The symbiotic fungus was inoculated at different stages of bag-cultured A. auricula to explain the relationship between the two fungi. The nutritional analysis of fruit bodies of A. auricula to explore the influence of the symbiotic fungus on nutritional structure of fruit bodies of A. auricula. [Results] The symbiotic fungus strain B-1 was identified as Lasiodiplodia theobromae. The inoculation experiment confirmed that the partial symbiosis between the A. auricula and strain B-1. The nutrition components of fruit bodies of A. auricula analysis found that it changed significantly in the case of the presence of strain B-1, and no obvious changed in morphology. [Conclusion] A. auricula and L. theobromae strain B-1 have the relationship of partial symbiosis, which is first report.