The optical phase retardation method is used in this paper. Based on the experiments on magnetic-field-induced Freedericksz transition of planar nematic liquid crystal, doped with trace bulin, fibre protein and double-azo etc, the trace adulteration effects on the transition of planar nematic liquid crystal are discussed; experimental curves and numerical calculation curves are given. Numerical calculation results are in agreement with measured results. It is shown that different impurities whose magnetic property, molecule shape and size are different, lead to different magnetic-field-induced Freedericksz transition, which is ulteriorly illuminated with the experiment on magnetic-field-induced Freedericksz transition of planar nematic liquid crystal doped with 2-benzothiazolethiol-linked porphyrinatozinc(Ⅱ).