Abstract: Through colonizing microbial communities by means of the polyurethane foam unit (PFU), we can evaluate water quality at the community level, and predict the ecological effeets of waste water. According to the MacArthur-Wilson equilibrium model, three function parameters (Seq, G and t90% ) were brought forward in the field ecological effect test and the laboratory toxicity test. In this article, the compound trapezoidal method and the least square method are used for ealculating the parameters. A BASIC program of the least square method has been made for the microcomputer TRS-80 and Sharp PC-1500. Either method can be used if an experiment is designed properly. The lack of fit test (LOF) can be applied to verify whether an experimental model is adapted to the theoretical model.