In the safety assessment of the actual CVQKD (continuous-variable quantum key distribution) system,the preparation measurement model is generally equivalent to the entanglement-based model,whose major drawback is that the shot noise variance is treated as a constant.As the attacks on the LO (local oscillator) from the Eve,the shot noise variance will change with LO.And in the process of safety analysis based on the shot noise variance calibration technology,there are loopholes in which the shot noise variance for calculating secret key rate is obtained by the linear relationship between the shot noise variance and the LO before distributing the quantum key.However,the shot noise variance is not accurate nor real-time.In the security analysis of system,all the noise parameters of the system are normalized to the shot noise variance.The Eve can reduce the shot noise variance by controlling the strength of LO,thus actual excess noise of system will increase.But legal communicating parties are still normalized based on previous larger shot noise variance,so that the excess noise of system is substantially underestimated.As a consequence,the Eve can obtain secret key information without attracting the attention of legal communicating parties by adopting some attacks, such as intercept-resend attack.Thus it is an essential factor for ensuring the system security to evaluate real-time shot noise variance accurately.In order to effectively resist the above mentioned attacks on the LO from the Eve,a scheme of CVQKD system based on real-time shot noise variance monitoring is presented to improve the security of CVQKD system.The shot noise variance calibration technology is adopted in this system.By adding the real-time shot noise variance monitoring modules to the primary CVQKD system,the real-time shot noise variance is assessed by the linear relationship between the shot noise variance and the LO.In the hardware system,independent clocks are adopted. Sampling in peak algorithm is applied to software system,and this effectively solves the problem that CVQKD system with LO clock source is at risk of shot noise variance calibration attack.The scheme prevents the hazards that the Eve changes previously calibrated linear relationship by regulating the pulse delay of the LO,and thus judges whether the system is safe through calculating the accurate and real-time secret key rate.The system can analyze the real-time security of quantum key distribution and display safety status of system.The experimental results show that this system can defend effectively the LO attacks from the Eve and improve the security performance of the CVQKD system.