Abstract: A barotropic model, including large-scale topography and constant zonal basic flow, was used to discuss the effects of large-scale topography on the low-frequency waves. The results show that the topographic factors which affect low-frequency waves are mostly maximun height of topography and topographic slope. The former makes frequency of topographic Rossby waves decrease, and the latter makes Rossby waves instable. Moreover, when topographic slope is optimum, it could also make Rossby wave turn into low-frequency waves. Constant zonal basic flow also plays a very important role in determining the instability of forced Rossby waves. In all zonal wave numbers, only the wave at large wave number (3 waves or larger) could be low-frequency waves.