OBJECTIVE： To improve the rational and scientific drug purchase quantity planned by hospital pharmacy. METHODS： The deficiency in present drug purchase module of hospital information system （HIS） was analyzed； the formula of planned purchase quantity was improved by incremental coefficient method， ABC analysis method and VEN analysis method. The high and low stored values and packaging volume were set to adjust planned drug purchase quantity. Based on the computer program， a one-key-getting procedure was quasi established， and five typical drugs were selected for simulation. RESULTS： Based on previous formula， the parameters representing change trend， incremental coefficient k1 and inventory coefficient k2 were added in new formula. The one-key-getting procedure could calculate the quantity of all drugs by simply entering the planning cycle N1 and the safe period N2 of the some category drugs into the system. After simulated operation practice， compared with pre-improvement， the occupation of funds was significantly decreased and the quantity of drug was rounded off according to package in post-improvement. CONCLUSIONS： Through function improvement of drug purchase module in HIS and the quasi establishment of one-key-getting purchase mode， planned drug purchase quantity is more reasonable， scientific and accurate. It ensures the clinical drug supply in time without causing a backlog， and effectively reduces the cost of manpower， material and capital.