Abstract: Horizontal earthquake effects on the protruding small towers on the top of tall buildings are investigated. Increscent coefficient is analyzed to adjust the horizontal earthquake effects on the tower by base-shear method. It is pointed out that the controlling factors of increscent coefficient include basic period of the main structure, period ratio and gravitational load ratio between the tower and the main structure. The increscent coefficient from Technical Specification for Concrete Structures of Tall Building is improved. Mode superposition method based on finite element model is used to compute seismic effects under 258 conditions. Based on the results, an increscent coefficient table is supplied for practical design.