Abstract: With the uncertainty of constraint condition of pile top, the lateral capacity of a single pile is hard to be evaluated precisely by theoretical computation or by routine static load tests when the horizontal earthquake action of pile foundations is checked. By investigating the calculation of the lateral capacity of a single pile of an existing building and testing through prototype tests, the lateral capacity of single pile under the constraint condition of pile top is acquired, which matches with the one under real working condition. The conclusion and methodology will be useful for the relevant calculation of lateral capacity of piles.