Feed rate of combine harvester was affected by many factors and difficult to be detected directly. A hydraulic stepless transmission system of threshing cylinder was designed, and the oil pressure was monitored. The oil pressure was used to represent the feed rate. By the engine bench tests, the relation equation between feed rate and oil pressure was obtained, and the distinguished method of oil pressure and threshing cylinder rotate speed which indicated feed rate was compared. Experiment results showed that the oil pressure could indicate feed rate timely and accurately, oil pressure was linear with the feed rate when the combine harvester was in the stable operation and crop properties were in the same. The threshing cylinder rotate speed could be considered as a constant.