The invention relates to a determination method for lanso chloride in lansoprazole. By adopting an impurity reference substance external standard method in a high performance liquid chromatography, a chromatographic column is an inverse extremely strong polar column, a moving phase is an acetonitrile-pH6.2-6.5 phosphate buffer, a diluent is an acetonitrile-pH10.5 phosphate buffer, and the detection wavelength is 210nm. The method provided by the invention is a mainstream high performance liquid chromatography (HCLP) which is extremely high in sensitivity. Experiments show that the limit of the detection is 0.5ppm and fully reaches the control demand that gene poisonous substances in the lansoprazole are 8ppm according to a Genotoxic Impurity Limit Guide, the specificity is good and the accuracy is high. Through methodology validation in light of an ICH guiding principle Q2b methodology, results meet the requirements of the guiding principle.