Abstract: Building undergo torsional excilations during an earthquake. But torsional earthquake input is not clear in building design by now. To estimate the torsional input of structures caused by earthquake, a simplified method is presented. The torsional ground motion, the torsional foundation input and their differences are discussed. It shows that the ratio of foundation length to the apparent earthquake wavelength and the ratio of foundation length to width affect the difference. A formula of torsional earthquake input of a structure is suggested for engineering structure purpose.