The advantages of free station would not be obvious in horizontal displacement monitoring of narrow and long Metro tunnel unless there are known references, as a result, incorrect deformation data would be obtained. The combination variance of unit weight test is a simple and effective method for determination of stable reference points of reference network. The practicability of the mentioned method is verified, and the influence of known reference points on the monitoring coordinate and the point position precision are analyzed. The results show that the monitoring precision can be improved, and the horizontal displacement can be correctly reflected by using free station method based on stable reference points.