The present invention provides a method of estimating water quality, an estimation apparatus of ultra-pure water using the same and a preparation system of ultra-pure water with the same, the said method of estimating water quality is simple and high sensitivity to evaluate the causticity of ultra-pure water as wash water for manufacturing the semiconductor or the liquid crystal with the infection of silicon as the target. The sample water such as ultra-pure water manufactured by the preparation system of ultra-pure water is contacted with silicon, the dissolved hydrogen concentration of sample water above is determineded, the improved part relate to the dissolved hydrogen concentration of sample water above is calculated. The said sample water is estimated whether it has the character ofcorrupt silicon surface base on the improved part of the dissolved hydrogen concentration.