The water flow movement basic equations of transient process caused by small fluctuation in the inclined roof tailrace system were deduced, and then analyzed and calculated. Afterward, the normal and basic laws found from basic data of a real power station were discussed. From the calculation and analysis, some conclusions were drawn as follows. If TW0 is fixed, the influence of slope i on the stability is minor when the slope i increases to a certain value during the transient process caused by small fluctuation, this result consists with the test appearances. If i is fixed, when TW0 is equal to or greater than 4.5 s, the curve of the fluctuation is a sustained oscillation till to a divergent one ， thus TW0 is suggested to be small than 4.0, this consists with the specification about TW0 in practical engineering. When other parameters such as B,y0,c0,H0 is decreased, the slope i must be increased in order that the dynamic quality of the system can be guaranted. All these above*.mentioned prove that the derived equations and mathematic model are right and reasonable. The method is easy to perform and the structure can cut down project investment.