Abstract: The injection system is a core component of the China Spallation Neutron Source (CSNS) accelerator. It is of great significance for improving beam power and stabilizing beam supply. The injection beam loss is one of the decisive factors for the operation of the rapid cycling synchrotron (RCS) at high power. In this paper, many sources of injection beam loss for CSNS accelerator were studied firstly, such as mismatch of injection parameters, choice of injection method, stripping foil scattering, non-stripped particle loss, and so on. Secondly, according to the beam commissioning process of the accelerator, the beam loss from different sources was adjusted and optimized to reduce the injection beam loss and improve the injection efficiency. Finally, a summary of the injection beam commissioning results was given. Preliminary measurement result shows the injection efficiency is approximately 99%. The improvement methods and suggestions are put forward to further reduce the beam loss and improve the injection efficiency.