Wind tunnel tests of single 3D hilly terrain for eight slopes were carried out, and the wind speed spectrum in different places and the effect of the hill slope were studied. The results show that the effect of the hill at the windward and hilltop far away from the hill can be ignored; at the leeward under the height of 0.8 h, the peak value of the wind speed spectrum increases and peak frequency moves to a high frequency. There is a single peak and the frequency bandwidth becomes narrower. It is assumed that the fluctuant wind speed energy at the leeward is composed of the inflow turbulence energy and the vortex energy induced by the hill, so the vortex energy can be separated. The calculation model of the vortex spectrum was proposed by the fitting of the wind tunnel test data. The wind tunnel test of different hill slopes shows that the slope 0.5 is the critical slope where the vortex will appear.