Abstract: All isochron methods require that samples have the same or close initial isotopic ratio and arethe product of the comagmatic suite and that the parent-daughter system is in a sealed state afterrock formation。 If the samples could not conform to the above-mentioned requitements , theisochron might be a fictitious or mixed ore ,and an incorrect isotope age would be given, In orderto distinguish the reasonable Sm-Nd isochron from fictitious or mixed ones ,the isotopic parame-ters ,T_DM and are intrbduced as judging factors. In the paper the three parameters are usedin analyzing the published Sm-Nd isotopic data ,and then the concepts of the concentrated samplegroup and concentration degree are proposed and the discrimination method , steps and basicsource program are introduced. Finally five examples of reasonable Sm-Nd isochrons and incorrectones are given , which indicates that the discrimination method is effective.