The service life of concrete was significantly dependant on chloride concentration in external environment. The electric flux of concretes in 6 hours at different NaCl solution concentration conditions was tested by ASTM method, and the influence of chloride concentration on chloride ion penetration coefficient for concrete was investigated. The results show that, a peak exists in the relation between chloride concentration and chloride ion penetration coefficient, which can be expressed in a form of quadratic polynomial. Danger range（about 40% to 60%, by mass） for chloride concentration will damage concrete durability mostly. The chloride ion penetration coefficient varies little with chloride concentration over 90%.
