<p>The theoretical security of continuous variable quantum key distribution (CV-QKD ) has been proved. However the gap between the theoretical model and the practical model can lead to the threat of the actual security of CV-QKD system in practice. An eavesdropper can use the imperfection of the homodyne detector to perform the saturation attacks. She can successfully obtain information without being detected by legitimate communication parties. In this paper, we analyze saturation attacks through atmospheric channels in CV-QKD system, and use mean-limited least square method to fit straight lines to defend against saturation attacks. Simulation results show that legal participants evaluate the information that has been eavesdropped because the secret key rate is negative.</p>