Abnormal sampling data is a main cause that results in mal-operation in power system. It is hard to distinguish abnormal sampling data from normal sampling data in fault circumstance under short sampling window. The character of electric current and voltage curves under normal and abnormal operation is firstly analyzed. It points out that the curves are continuously derivable except for several discontinuous points and the derivative is piecewise continuous. This character can be used to judge whether the sampling data is valid or not. As the sampling step is relatively fixed， the derivability can be judged by the backward and forward differences of continuous sampling data. Furthermore， sampling data are judged by setting reasonable differential error threshold. Then the method is briefly called sampling data judging method with three continuous valid data. The solving method for differential error threshold and analysis of several practical problems are given. Simulation results prove the feasibility of this method.