In order to ensure high-yield of horizontal gas wells, it is significant to accurately predict the emptying condition of the liquid loading, as well as the liquid covered range of the gas well holes. A visualization experiment on the liquid loading characteristic in horizontal pipes was performed using air and water as the gas and liquid phases to test a rectangular cross-section pipe, respectively. From the experiment, the relationship between the gas flow parameters and the liquid loading in horizontal pipes and the shear factor of the gas–liquid interface under the liquid loading condition was obtained. The experimental results show that for a horizontal pipe the existence of liquid loading corresponds to a wide range of the gas velocity in the pipe. Within the gas velocity range, the length of the liquid loading, or the liquid coverage area in the pipe, decreases with the increase of the gas velocity. Under the liquid loading condition, the interfacial shear factor increases with the increase of the gas phase Reynolds number, while the increase is not significant. Meanwhile, the theoretical prediction model of the liquid loading characteristic in horizontal pipes was also studied. Based on the mechanism analysis of forming an inclined liquid surface and the force analysis of the gas and liquid flows, a basic equation was established to depict the liquid loading characteristic of horizontal pipes. Then, the model was closed by the envelope principle. Thus, combined with the experimental correlations of the interface shear factor and the wall shear factor obtained from the experiment, a closed theoretical prediction model was proposed for predicting the liquid loading characteristic of horizontal pipes in engineering applications. Predicting results of the model show a good agreement with the experimental results. Therefore, this model can be used to predict the emptying condition of the liquid loading in horizontal pipes, as well as the liquid covered range of the horizontal gas well holes.