Synthetic microbial community (SMC) hold immense potential for application across various vital fields. However, the rational construction of SMCs still faces considerable challenges. The function of SMC is contingent upon the function of individual species within it and their interactions. But the relationship between the function of the SMC and the functions of the individual species it contains, which we call the ‘interaction effect’, is still unclear. In this study, we categorize the interaction effects of species in SMCs into seven types by comparing the functions of SMCs and their constituent species. These types include linearly related additive effect (AE), non-linearly related synergistic effect (SE), stimulatory effect (STE), strong effect (STRE), interference effect (IE), weak effect (WE), and inhibitory effect (INE). This classification of interaction effect can be used to assess the efficiency of SMC. It is not easy to quickly construct efficient SMC with synergistic effects (SE), although numerous reports document synergistic effects (SE) in SMCs. Moreover, this classification of interaction effect may also apply in other fields such as ecological communities, drug combinations, composite materials.