The work aims to study the erosion and corrosion of Q235 steel in environment with different flow rates and sand contents. The rotary erosion-corrosion tester was used to study the effects of flow rate and sand content on erosion-corrosion behavior through electrochemical test methods, surface microscopic analysis and weight loss measurement. The surface of the sample mainly includes corrosion pits and scratch. The number of pits increased with the increasing flow rate. The wear of the sand caused obvious scratches on the surface of the sample. When the sand content was small, the pits on the surface of the sample were large and relatively scattered. As the sand content increased, the number of the pits on the surface of the sample increased, but the diameter of the pits decreased. As the flow rate increased, the corrosion product film on the surface of the sample became denser. As the sand content increased, the corrosion product film on the surface of the sample became thicker, and more stable Fe2O3 appeared. When the seawater flow rate and the sand content were small, the erosion and corrosion of Q235 was electrochemically characterized as single-layer structure corrosion product film. With the increase of flow rate and sand content, the electrochemical characterization was transformed into a corrosion product film of double-layer structure, and the sand particles could not directly act on the surface of the substrate. When the flow rate increased from 1 m/s to 5 m/s, the erosion-corrosion rate increased from 0.0113 mm/a to 0.0309 mm/a, and the maximum pit depth of Q235 steel increased from 34.47 μm to 281.94 μm. When the sand content increased from 0.15wt% to 1wt%, the erosion-corrosion rate changed from 0.0113 mm/a to 0.0107 mm/a, and the maximum pit depth changed from 34.47 μm to 16.41 μm. The erosion-corrosion rate and corrosion pit depth of Q235 steel are sensitive to the eroding flow rate, but less sensitive to the change of sand content.