The large amount of microplastics in the aquatic environment has increasingly attracted wide attention. Studying the relevant laws and distribution of microplastics in wastewater treatment plants is crucial for solving microplastics pollution in aquatic ecosystem. However, there are many biological samples in the sewage of the waste water treatment plant, which pose great challenge in the detection and counting of the microbes. Therefore, it is necessary to apply pre-processing techniques. In this paper, we studied five digestion methods, namely KOH, NaOH, H2O2, HNO3·HCl and HNO3. Afterwards, the influence of different digestion methods on the digestion effect, recovery and surface morphology of simulated plastics were investigated. At the same time, the most suitable microplastics digestion method was selected by orthogonal experiment. The results showed that the digestion effect of hydrogen peroxide was much better than that of other methods and it had the least influence on the recovery of microplastics. In addition, there was no apparent effect on the surface morphology of microplastics and their recovery rate could reach 96.7%±0.3%. Therefore, the results proved that this method is an effective pretreatment technique suitable for determining microplastics in sewage.