In order to study the effect of longwalI mining on suface stream water,monitoring stations of water flow rate was established.A lot of water flowing data were collected before,during and after longwall mining.Based on monitoring data,the effects of longwall mining on surface stream water were analyzed.The results demonstrate that longwall mining has effects on the surface stream water:and the stream water would be lost and decrease due to longwall mining but never go into underground through fractured zone.Also.the mechanism of water loss due to longwall mining was presented.The stream water can go into the surface cracks in the intersection of stream and surface cracks.longwall mining subsidence can change the surface stream slope and the downstream water flowing status.The results also show the effects of longwall mining on stream water are temporary and about one or two years later,surface stream water can be recovered.