Wettype microwave sterilization is a newly developed technology. A 3D unsteady active heating mathematical model of hot water and microwave sterilization in a single mode system at the frequency of 2450MHz was established based on the coupled equations of electromagnetic heating and heat transfer. The distribution of electromagnetic field in microwave cavity and heating pattern in food sample were obtained by using finite element method and the good results were used to guide the design of microwave sterilization equipment. It was determined that the optimum size of pyramidal horn was 283.92mm×136.50mm×109.20mm and the height of cavity was 75mm. A twocavity microwave sterilization system was developed. Experimental results demonstrated the temperature of package food rose from 50℃ to 142℃ in just 2min. Furthermore, it showed that the heating pattern in food was stable and the temperature distribution was more uniform owing to water circulation which reduced the edge overheating effect. In addition, this study confirmed that it not only effectively met the requirements of pasteurization process but also verified the effectiveness and rationality of the designed device.