Particles with a single-size gradation were widely used to investigate the erosion behavior of steel, which can not well simulate the service environment of steel components in practice. For simulating the service environment of steel components in hydraulic engineering, carbon steel was eroded in sand and mud mixed solutions in the present study. The erosion behavior of steel was investigated by electrochemical test weight loss measurement in sand and mud mixed solutions, and the surface morphology of the eroded surface was observed. Coarse sand, fine sand, and mud with the median particle size (D50) of 0.775 mm, 0.270 mm, and 0.012 mm were adopted, respectively. The results suggested that the erosion of the carbon steel was aggravated by the increasing rotational speed in these sand and mud mixed solutions. The erosion was obviously suppressed by the addition of mud in the solution containing coarse sand at a high rotational speed (≥750 rmp), while those of counterparts initially enhanced and then decreased with the increasing mud content in the same coarse sand solution at a low rotational speed (500 rmp). However, the erosion of the carbon steel did not show any significant influence by the addition of mud in the fine sand solutions. The reasons for the different situations also were discussed.