The influence of imperfect Faraday mirror on secret key rate of continuous variable quantum key distribution system in reverse reconciliation is considered with theoretical analysis and numerical simulation. It takes entanglement based scheme. It′s found that Faraday mirror′s imperfection reduces the secret key rate and transmission distance of system. The impact increases if the imperfection turns more serious. Besides it finds using great modulation variance can reduce the impact of Faraday mirror significantly.