A prototype 3D simulation system for reactor decommissioning based on the software of DELMIA, and VIRTOOLS is built. This system is composed of the database, real simulating scene, decommissioning process simulating, decommissioning data analyzing, project management, evaluation and expert assistant decision-making and other sub-systems. On the basis of this simulation system, 3D radiation field computation algorithm is developed based on the point-kernel method and the computing errors of the radiation level at key points are less than ten times of the measuring data. The computation software of 3D radiation field is programmed based on the software of VS and SQL server. This software is built in the system and dynamic calculation and data refresh of 3D radiation field are realized. The mapping formula between color and radiation data is proposed and visual display of 3D radiation field is realized. The calculating method of person exposure dose is worked out based on the decommissioning procedures by acquiring related parameters and real-time display of the worker's dose is realized. Virtual cutting algorithm is developed by segmentation and reorganization of three digital models. Cutting process can be simulated for any pipes at any position.