Thermal management is crucial to spacecraft design in that most of the subsystems are electronic devices, and electronic devices need to work in a specific temperature range to assure the performance. To meet the requirements of spacecraft thermal management, the engineers always need a convenient and useful tool to assist with the design process. Thermal Freetop is an outstanding fit for this purpose. Thermal Freetop is a finite element thermal analysis package with the similar interfaces and same functions as those of Thermal Desktop, a commercial spacecraft thermal management software tool,and Thermal Freetop is based on the open source software such as FreeCAD, Salome, Paraview, 42, GMAT, ElmerSolver and ElmerGrid.