Factors influencing chloride penetration depth as determined by AgNO3 colorimetric method were investigated, through analyzing a series of rapid chloride migration(RCM) test results. It is revealed that, compared with the true value, AgNO3 colorimetric method underestimates the chloride penetration depth. And correspondingly, the migration coefficient calculated by the RCM method is smaller than the true value. The chloride concentration at the color change boundary has a significant influence on the calculated chloride migration coefficient. According to the experimental results, this factor could bring an error up to 20% to the calculation. The chloride concentration at the color change boundary is a function of calcium hydroxide content in cement based materials. The higher the calcium hydroxide content is the higher the chloride concentration is required for color change. The incorporation of fly ash could make the chloride concentration required for color change lower, thus the chloride penetration depth as measured by AgNO3 colorimetric method tends to be larger as compared with the case of plain cement concrete.