Abstract: The rapid cycling synchrotron (RCS) of the China Spallation Neutron Source (CSNS) is a high intensity proton accelerator, which accumulates the 80 MeV proton beam and accelerates it to 1.6 GeV with the repetition frequency of 25 Hz. The designed beam power is 100 kW. The beam commissioning of RCS began in May, 2017 and reached the target beam power of 100 kW in February, 2020. Two buckets are perfectly filled with two bunches in the RCS. The two bunches are extracted from the RCS within one turn by the kicker magnet and the rise time of the kicker field is 256 ns after the manufacture with the kicker field trigger and jitter error of about 20 ns. The revolution time at extraction energy is 818 ns in the RCS. The bunch length is the main parameter of the beam dynamics and its maximum of about 130 ns is allowed without beam loss at extraction. The measured bunch length is accurate (105±1) ns in the operation, which is consistent with the simulation result. A forthcoming linac energy upgrade will allow the beam power to reach 500 kW by increasing the linac beam energy to 300 MeV in order to decrease the space charge effect at low energy and increase the beam current intensity in the RCS. The final bunch length of 500 kW may be bigger than the allowed value, so it must be compressed. The bunch length can be compressed by enhancing the cavity voltage in theory and the bunch length as a function of the voltage curve was simulated in the paper. The bunch length can be compressed by enhancing the cavity voltage in the last half of the acceleration cycle. Without beam loss, the voltage at extraction should be enhanced to 140 kV and the bunch length is about 120 ns. Based on the state of 100 kW beam, the bunch length compression method was tested under the condition of keeping the cavity stable operation and the bunch length can be compressed about 20 ns by enhancing the cavity voltage in the last half of the acceleration cycle. The simulation and experiment result shows that the bunch length compression method by enhancing the voltage is the effective way to avoid the beam loss at extraction for the RCS with beam power of 500 kW.