The calculation method for bedload transport rate is studied by flume experiment, through which the effect of different flow intensities and bed material compositions on the transport rate of continuous and discontinuous broadly-graded bedload is investigated. A formula of relative flow intensity in consideration of non-uniform sediment is set up and regression analysis is carried out by using the experiment data. The relationship between relative flow intensity and bedload transport rate is further analyzed. This formula is proved to be rational as it reflects the effect of non-uniform discontinuous broadly-graded bed material on sediment transportation.