OBJECTIVE： To explore losses and gains （L&G） and L&G ratio induced by Essential Medicine System in a county. METHODS： By choosing a county in western China as sample area， field investigation was used to collect outpatient and inpatient visits， outpatient and inpatient income， drug income， total length of stay and medical insurance reimbursement criteria in primary medical institutions （township health centers， village health rooms） of the county during 2009-2015. By setting the year 2009 as the baseline year， the drug cost reimbursed by medical insurance was simulated and calculated when Essential Medicine System were not implemented； L&G and L&G ratio of medical insurance were calculated by comparing with actual drug cost reimbursed by medical insurance. RESULTS： The year 2012， in which the sample county fully implemented the Essential Medicine System was the turning year. Medical insurance funds lost in primary medical institutions of the county during 2010-2011（lost    437 000，915 000 yuan， respectively）， but gained during 2012 to 2015（gained 199 000，494 000，858 000，1 290 000 yuan， respectively）； the L&G ratio increased from －0.67％ to 1.21％. For reimbursed outpatient drug cost and inpatient cost， L&G of medical insurance were different. For reimbursed drug cost of village health room and township health center， L&G of medical insurance were also different. CONCLUSIONS： The implementation of Essential Medicine System benefits to medical insurance within the county and Medical insurance funds can be saved.