Efficiency monitoring is an effective means of hydraulic turbine state monitoring. In order to express the monitoring results visually and comprehensively， an on-line efficiency monitoring system based on surface of efficiency characteristic is introduced. The surface of efficiency characteristic， which is gained by interpolation from contour curve of efficiency， plays an important role in the monitoring software. Based on the parameters of circumstance needed to gain， a data acquisition and processing board which can consecutively sample is developed. The monitoring system， which has behaved well in model experiments and live experiments， expresses the circumstance data visually and comprehensively， and is easy to operate.