Abstract：In order to confirm An-Jialing open-pit coal mine’s economical and reasonable length of expansion working- line, this paper established 3D geology and mining model of An-Jialing open pit coal mine by using 3Dmine mining software. 5 working-line length programmes after the first mining area were selected to simulate mining operation. Each programme has 17 engineering stages. Calculating data including the amount of coal and rock, stripping ratio and haulage distance by comparing and selecting better mining schemes was used to confirm the programme of economical and reasonable working-line length. Research result shows that An-Jialing open pit coal mine’s economical and reasonable length of working-line is 1 500 meters.