To study the vertical ground motion characteristics of Wenchuan earthquake, a vertical ground motion attenuation relationship was fitted. First, the site effect and hanging/foot wall effect on the vertical ground motion were investigated, and the characteristics of vertical pseudo-spectral acceleration (a_PS, damping ratio 0.05) were analyzed. Then, a simple vertical design response spectrum was proposed. The vertical-to-horizontal ratios (V/H) relationship was obtained by using vertical and horizontal ground motion attenuation formulas, and the characteristics of V/H were analyzed at different distances in different periods. Results showed that the vertical ground motion was affected by the site conditions and the hanging/foot wall. There were many differences between the vertical and the horizontal response spectra, including the excellent period, the characteristic period, the period span of oscillation of peak response spectrum, and the attenuation rate of the falling phase. V/H had great discreteness, which showed completely different characteristics with the variation of distances in different periods, whereas the site effect and the hanging/foot wall effect on V/H were caused by the difference between those on vertical and horizontal ground motions. The V/H of acceleration response spectrum exhibited a "saddle type", while it had different characteristics at different distances that the maximum value appeared in high frequency band (t<0.1 s) for near field and in low frequency band (t>1 s) for far field. Considering the process from research to application, the investigation of vertical ground motion in China in the future, on one hand, shall continuously improve the vertical design response spectrum, and on the other hand, shall consider estimating the key value of response spectrum by using the relationship of V/H.