The invention relates to the technical field of soil and water conservation research, and provides an exposed stone runoff collecting device and a runoff coefficient measuring method. The exposed stone runoff collecting device includes a runoff collecting tank and a runoff collecting cylinder; the runoff collecting tank has an annular structure and matches the vertical projection shape of the bottom of the exposed stone to be measured, and is used for collecting the rainfall runoff flowing through the surface of the exposed stone to be measured; the runoff collecting cylinder is connected to the runoff collecting tank through a connecting pipe for collecting the rainwater collected in the runoff collecting tank. According to the exposed stone runoff collecting device and the runoff coefficient measuring method provided by the invention, the runoff collecting tank matching the vertical projection shape of the bottom of the exposed stone is provided, and the rainwater collected in the runoff collecting tank flows into the runoff collecting cylinder; therefore, the rainfall runoff of the entire exposed stone can be accurately collected, and meanwhile, the runoff collecting cylinder can effectively store the rainfall runoff resources and provide water source guarantee for agricultural irrigation; and the use of the exposed stone runoff collecting device to collect the exposed stonerunoff is beneficial to improve the measurement accuracy of the exposed stone runoff coefficient.