There are many low-pressure gas wells in domestic gas field, this kind of wells have low production, and serious liquid loading problem. Various technologies have utilized to prevent this problem in the area, but the liquid loading mechanism should be definite firstly to investigate the liquid unloading process optimization and applicability. In this paper, we reviewed the mechanism related to liquid loading for gas well, including gas-liquid two phase flow in vertical, inclined and horizontal circular pipe, critical liquid unloading and the criteria, liquid loading in borehole and the forecasting model for it, and the related experimental apparatus and method, as well as proposed the questions needed attention and related issues to be resolved in this subject.