Abstract: The surface outlets at Ertan hydropower plant are arc gates, and the hoisting devices are duplex hydraulic hoists.It was good since operation, and long- distance automatic control hoisting has taken the place of on- spot manual hoisting in flood period. In the rise- and- fall process, due to different operation situation, asynchrony would appear. As a result, rectification circuit was considered in the design. This paper introduced the principle and fault analysis of the duplex hydraulic hoist at surface outlets.