Coastal concrete structures have very tough working environment with combined action of loads, chloride ion erosion and so on. According to the cores drilled from the concrete of one existing sluice pier along Oujiang river estuary region in Wenzhou, the mix proportion of the concrete was determined. Based on measured results of chloride concentration, the erosion regularity of chloride ion in the existing coastal concrete was analyzed, and the diffusion coefficients of the existing concrete and initial corrosion time of steel bars in the concrete were calculated by Ficks second law and the Monte Carlo method. The results show that obviously there exist the convective zones of chloride ion erosion in the 5350d concrete with thicknesses of exceeding 20mm. Under the various actions, the resistance to chloride ion erosion in existing coastal concrete is reduced sharply, and the critical concentration range of chloride ion for steel bars corrosion is 012% to 018%, and the initial corrosion time is merely 3000 d. The results show also the Monte Carlo method is a feasible method for analysis of the initial corrosion time of steel bars in the concrete in the nature chloride ion environment.