The invention discloses open-cast coal mine and a mining method thereof. The open-cast coal mine comprises lead ore and lagging ore which are arranged adjacently, wherein the lead ore comprises a first mining region and a second mining region which are arranged in parallel, the lagging ore comprises a third mining region and a fourth mining region which are arranged in parallel, the inner dump working line of the first mining region is adjacent to the stope mining region of the fourth mining region, the inner dump working line of the second mining region lags the inner dump working line of the first mining region, and the stope working line of the fourth mining region lags the stope working line of the third mining region. By adjusting the stope working line and the inner dump working line of the adjacent ore regions to be in a polygonal line shape, the distance between the lead ore inner dump working line and the lagging ore stope working line at juncture of the adjacent ore regions is increased, and the completeness and smoothness of the end slope transportation line at the juncture of the adjacent ore regions are ensured, so that the transportation equipment is convenient to arrange, the transportation cost is lowered, and the economical benefit and safety of the opencast pit are favorably improved.