The redefinition of the unit of mass by tracing kilogram to Planck constant has been widely accepted in the field of metrology. In China, Joule balance method is proposed by NIM to measure the Planck constant and redefine the kilogram. Compared with the Watt balance method in other countries, Joule balance method benefits from the advantage of static measurement. The whole system experiments for the first generation of the Joule balance system have been finished. The whole measurement uncertainty was analyzed in detail, which will help us to further optimize the design scheme and improve the experiment process. Finally, we obtained that the whole measurement uncertainty is 2.5×10-6. Aiming at the major sources of the measurement uncertainty, several improvement methods are put forward for the next generation of the Joule balance system, which provides the ensuring measure for further reducing the measurement uncertainty.