OBJECTIVE： To discuss the effects of construction and application of the prescription checking beforehand system on rational drug use in outpatient department. METHODS： According to the demands of outpatient pharmacy， related module of previous HIS was revised to develop prescription checking beforehand system； the system was used to check outpatient prescription beforehand and related supporting measures were implemented. The changes of related evaluation index were compared before （Apr.-Sept. in 2014） and after （Apr.-Sept. in 2015） the implementation of prescription checking beforehand system. RESULTS： The rationality of outpatient prescription could be checked before payment in our hospital through establishing prescription checking module， formulating workflow of prescription checking system and conducting prescription intervention management. Compared with before the implementation of the system， utilization ratio of oral antibiotics， utilization ratio of antibiotics for injection and the rate of irrational prescription decreased from 25.7％， 31.5％， 5.10％ to 16.5％， 19.8％， 1.98％. Successful rate of irrational prescription intervention increased from 24.3％ to 85.5％. The incidence of ADR induced by wrong prescription， the rate of overproof medical insurance cost and the rate of outpatient complaint decreased significantly （P＜0.000 1）. CONCLUSIONS： The construction of prescription checking beforehand system can promote rational drug use and hospital service.