A high-performance liquid chromatography (HPLC) method based on 1-phenyl-3-methyl-5-pyrazolinone (PMP) as an agent of pre-derivatization was developed for the accurate determination of lactose content in clozapine tablets. The results showed that a good linear relationship between the peak area of the derivative product and lactose concentration was observed in the range of 0.060 4~0.399 7 mg/mL, the linear correlation coefficient R2 was 0.999 4. The recovery was within the range of 96.7%~99.2% and the limit of quantification was estimated to be 0.32 μg/mL. Three batches of clozapine tablet imitation samples were randomly selected for testing, and the lactose contents were 42.09 mg/tablet, 43.36 mg/tablet and 43.45 mg/tablet, respectively. The deviation between the measured results and the prescribed content was less than 2.0 mg/ tablet, and the results could fully meet the requirement of detection of lactose in drugs. Moreover, compared with the existing tests, this method had the advantages of high specificity, high sensitivity, accurate and reliable results, and could provide data support for the development of clozapine consistency evaluation prescription, which is beneficial to the acceleration in the development and success rate of generic drugs.