In order to investigate the onset law of muddy clay, the onset characteristics of muddy clay with different sedimentary densities were investigated experimentally by the synchronous measurement of water grey level and flow velocity. The experimental results show that the phenomenon of the onset of muddy clay can be divided into weak onset and general onset. To the weak onset, the rate of erosion on the muddy bed is low and the variation of water grey level is about 10. While to the general onset, the rate of erosion corresponding to a sudden change in the curve of water grey level increases evidently and the variation of water grey level is usually more than 20. The flow velocity of the general onset is 1.5 times as much as that of the weak onset.