This paper reports for the first time that the MBP fermentation is based on the relationship between the two selected good strains of G. Candidum As.2.361 and Aspergillus niger No.303, which is commensalism. The floras, like the appearance of hybridization formed by As. 2.361 when it is co cultivated with No.303 in 4320 MBP fermentation, are proved to be a specific feature of commensalism through Fusant Test, Heterocaryon Test, Substrate Mycelia Observation, Diffusion block Test and Microscopic Examination. The quantity of floras directly reflects the extent of commensalism and the quality of MBP 4320 as well, which is of great application value. Effective factors in commensalism are citric acid, protease, glucan amylase, and a amylase produced by No.303, and they function directly, indirectly and cooperatively. Among the factors, amino acids, especially Glu, catalyzed by protease from No.303 and citric acid produced by No.303 can stimulate growth of No.2.361 strikingly. They are the two newly found top commensal factors in producing MBP from starch materials, which is discovered through orthogonal test and is completely different from other reports that the main factors in producing SCP with starch materials are such enzymes as glucan amylase.