Considering that perfect channel state information(CSI)is hard to obtain in practice,the capacity of downlink distributed antennas system(DAS)with imperfect CSI is analyzed over Rayleigh fading channel.Based on the performance analysis,using the probability density function and numerical calculation,an accurate closedform expression of ergodic capacity of downlink DAS under imperfect CSI is derived.It includes the one under perfect CSI as a special case.This theoretical expression can provide good performance evaluation for downlink DAS for both perfect and imperfect CSI due to its accuracy.Simulation results indicate that the theoretical analysis agrees well with the corresponding simulation,and the capacity can be increased effectively by decreasing the estimation error and/or path loss. Considering that perfect channel state information (CSI) is hard to obtain in practice, the capacity of downlink distributed antenna system (DAS) with imperfect CSI is analyzed over Rayleigh fading channel. Based on the performance analysis, using the probability density function and numerical calculation , an accurate closed form expression of ergodic capacity of downlink DAS under imperfect CSI is derived. It includes the one under perfect CSI as a special case. This theoretical expression can provide good performance evaluation for downlink DAS for both perfect and imperfect CSI due to its accuracy . Simulating results that that the theoretical analysis agrees well with the corresponding simulation, and the capacity can be higher effectively by decreasing the estimation error and / or path loss.