Abstract: The effeet of terrain on large scale steady atmospheric motion in tropics is studied for uniform and non-uniform basic flows. The analytic formula of orographic influence function is derived from the lineanized barotropic primitive equation on equatorial β-plane. The analysis shows that the forced atmospheric motion depends not only on the direction of the basic flow which crosses over the mountain,but the latitudes and scale of the mountain as well. As moderate westerly crosses a mountain; orographic troughs form in the leeside. It seems that some kind of non-uniform basic flow may make the meridional extent of orographic forced motion confined.Using this model,the pressure and flow patterns near the equatorial area of south east Asia are simulated the result qualitatively consists with the observed features,Finally,the formation mechanism of the channels of cross-equatorial curent is discussed.