By discussing genesis of sphalerite RbSr isochron, it can be pointed that the mixing line produced by the tested isotopes system, which is cogenetic, syngenetic and closed, brings the geological age. The system related to tested isotopes, however, which may be no cogenetic, syngenetic or closed, also brings "isochron" —— a meaningless mixing line. Sphalerite, the object of isotope dating, maybe just play the role of "container", while mineral particulates such as carbonate, silicate etc. produce remarkable influence to the analyses, so a elaborate mineralogical work is indispensable before isotopes analyses carrying on. So the isochron is bad to reflect the geological fact if these particulates are not discussed on genesis or the dating object has not been tested on other isotopic chronological methods.