In the rapid cycling synchrotron of the China Spallation Neutron Source, the anti-correlated painting was adopted for the design scheme of the injection system. In the beam commissioning, with the optimization of the anti-correlated painting, the injection beam loss has been well controlled and the injection efficiency has exceeded 99%. Combined with other aspects of adjustments, the beam power on the target has reached 50 kW. In this paper, we have studied the injection optimization in the beam commissioning. Compared to the simulation results of the design scheme, the transverse beam distribution, transverse coupling effect and beam loss of the anti-correlated painting in the beam commissioning are somewhat different. Through the machine studies, we have carefully analyzed these differences and studied their reasons.