The environment of the tidal zone is very complex. The interactions of dry-wet alternation and sea erosion lead to serious corrosion of steel structures, which makes it difficult to adopt protective methods. Therefore, it is of great significance to study the corrosion and protection methods of steel in tidal zone. At present, the widely used protection method in tidal zone is coating which is effective in short term. However, it is easy to cause blister failure during the long-term service process, and it will increase the maintenance cost. Sacrificing anode protection is the most common method used in the seawater environment due to its advantages such as low cost, simple operation, no external current, no interference with adjacent metal facilities, good current dispersion ability, easy management and maintenance and high efficiency, etc.. However, in the tidal zone, sacrificial anode protective is effective only when the protected metal is in seawater immersion state. After the tide receded, the protected metal exposes in air. At this time, the current loop is destroyed, and the sacrificial anode protection effect is weakened. Therefore, it is commonly known that the sacrificial anode protection method can not protect the whole tidal zone against corrosion. At present, the corrosion process and mechanism of steel structures under sacrificial anode protection in the tidal zone are not clear. In order to study the corrosion mechanism of sacrificial anode protection, a corrosion experimental trough was designed to simulate the tidal zone and immersion zone. The electrode potential of Q235B mild steel under different protecting area of sacrificial anode in it was monitored in situ by the electrochemical workstation. The results show that under the sacrificial anode protection, the long scale specimen of Q235B steel is protected well, the corrosion degree in the tidal zone gradually reduces with the decrease of tide level, and the protected height increases with the increase of sacrificial anode area. Protective effect of sacrificial anode is mainly decided by the IR drop of specimen surface when the steel structures are exposed in the air, the smaller value of IR drop, the better protection effect. However, although the protection effect of steel structures can be improved by increasing the metal area of sacrificial anode, sometimes a part of steel structure may be in the state of excessive protection. The effective way to solve the corrosion problem of tidal zone needs to cooperate the sacrificial anode with other protective methods.