In order to investigate the topographic factor of single hill with different slopes and the influence of wind flow over hill on transmission tower-line system, wind tunnel tests of three hill models with different slope were carried out and the results were compared with several different codes in China and abroad. The topographic factor at hilltop and along the hillside were analyzed, together with the vertical wind component and turbulence of wind field. The results show that the topographic factors at hilltop for different slopes are the largest in the Load code for the design of building structures and Code for design of high-rising structures. The topographic factor at hilltop does not always increase with the increasing slope of the hill，and it gradually decreases when the slope increases from 0.577 to 1. Additionally，The topographic factors of cross-wind at the surface of the hill calculated by the existing fitting formula are smaller than the measured ones, while the calculated values in the upper area are larger than the test one. Moreover, the maximum value of the vertical wind speed component decreases as the slope of the hill increases, and the ratios between the maximum vertical wind speed component and the inflow velocity are 0.523 and -0.542 for the windward and leeward side of the three types of slopes, respectively. The turbulence at the surface increases with the increasing slope of the hill, which is most obvious at the leeward side of steep hill. For example, when the slope is 1, the turbulence at the leeward side of the hill reaches to 35% at the 0.6 times height of the hill.