The invention relates to a preparation method of benzazepine compounds, particularly a preparation method of 7-chloro-2,3,4,5-tetrahydro-1H-1-benzazepino-5-ketone. The 7-chloro-2,3,4,5-tetrahydro-1H-1-benzazepino-5-ketone can be used as an intermediate for preparing a pitressin antagonist medicament Tolvaptan. The method comprises the following steps: by using parachloroaniline as a raw material, reacting with paratoluensulfonyl chloride in a condensation mode under the alkaline condition to firstly protect amino groups; under the alkaline condition, coupling with ethyl 4-bromobutyrate to obtain a compound; in the presence of alkali, hydrolyzing the obtained compound, and acidifying; in dichloromethane, preparing acyl chloride from the acidified compound under the action of thionyl chloride; under the action of Lewis acid, carrying out Friedel-Crafts acylation reaction to perform intramolecular cyclization; and finally, removing tosyl groups to obtain the target product. The invention has the advantages of simple technique and low cost, and is convenient for industrial large-scale production.