Quantum key distribution (QKD) has become an alternative technology defensing the security threat resulting from quantum computing. Owing to its superiority in coexistence with optical infrastructures and cost-effective, continuous variable QKD (CV-QKD) is widely studied and developed towards the practical application stage. The standardized measurement of critical parameters of the system should be studied urgently.In this work we propose a scheme based on balanced homodyne detection with phase-randomized local oscillator to realize the average photon number measurement. By controlling local oscillator phase positively, which obeys \begin{document}$ [0,\mathrm{ }2\mathrm{\pi }] $\end{document} uniform distribution, a linear relationship between average photon numbers and the variance of detector output can be obtained in simulation and experiment. With high-accuracy (～0.1) photons/pulse and large range ～23 dB, this average photon number measurement scheme provides a great reference to evaluating the critical parameters of CV-QKD system in the practical process.
