Abstract: For production test of 300 m fully mechanized working face of caving coal in Lutian coal mine of Haibowan mining Co., Ltd. of Shenhua group, FLAC3D was used to determinate reasonable size of crossheading pillars affected by mining. Through establishing model and simulation, conclusions are as following: along with lengthening of working face,periodic weighting eases,so as to reduce impact to crossheading pillars to some extent; coal pillars which are influenced with the maximum range are 100~110 m away from the rear area of working face, laterally significant affect range is about 10 m, and depth of plastic damage is 7 m; because protection pillar will be influenced by next working, the size of the protection pillar of super long working face with shallow depth under the same geological conditions is 15 m.