A new viewpoint on the accelerating mechanism of accelerators has been suggested by using XRD, combinedwater measurement, hydration liquid phase analyses, DTA and ect.. The result shows that, alkali environment, which is created by adding accelerator, helps gypsum retard hydration of C 3A, but accelerates C 3S hydration. High alkalinity can accelerate cement hydration only when it reacts with cement, skeleton of hydrated aluminate calcium is formed quickly. And then accelerator accelerates hydration of C 3S, and large quantity of CSH gel fill that skeleton. So, concrete is setting and the structural strength is formed.