Abstract: Hydralazine-N-lactose is a main impurity in hydralazine tablets. Hydralazine and lactose were used as raw materials, and silica gel column and preparation column of octadecylsilane bonded silica were selected to prepare and purify hydralazine-N-lactose. Its in vitro metabolites were identified by utilizing SD rat liver microsomes. The results of samples in vitro were compared with blank group and the resulting metabolites structures were identified by LC-MS/MS. Hydrazine-N-lactose produced six main metabolites, including s-triazolo［3, 4-a］phthalazine, 3-methyl-s-triazolo［3, 4-a］phthalazine, 3-(1-hydroxy)methyl-s-triazolo［3, 4-a］phthalazine, 3-hydroxymethyl-s-triazolo［3, 4-a］phthalazine, hydralazine and dehydro-hydralazine-N-lactose after in vitro metabolism. This study will be valuable in hydralazine-lactose conjunctions metabolism research and hydralazine formulation safety.