A radiation therapy planning system (RTPS) is medical device software, which is used to simulate the application of radiation to a patient for a proposed radiotherapy. RTPS is important component in radiotherapy solution. Good development practice is key factor that assures RTPS quality. Based on a RTPS product development project, this paper gives a life cycle model of medical device software development. The model divides RTPS development process-into five phases, which is project feasibility study, project planning and design, project execution and implementation, project verification and validation, and project support and maintenance. And also discusses the activities in each phase.