With Xiaoguanzhuang iron mine as a case study, we investigated the influence of deep geostress field, in particular, the influence of maximum horizontal stress on rockmass displacement caused by deep mining. Based on in-situ stress measuring results, we quantitatively analyzed the distributing law of the in-situ stress field in the mine. To provide basis for the support of surrounding rockmass of the roadway, we monitored the rockmass deformation and made detailed measurements at two adjacent mining areas. Results showed that the geostress field was dominated by horizontal stress, which also determined the mining activity and its effect. The orientation of maximum horizontal stress was influenced by geological structure. Deformation of the roadway was also affected by the angle between the orientation of maximum horizontal stress and the axis of roadway. Other stress control measures, including artificial support, are briefly discussed. It's concluded that proper orientation of sublevel roadway is the key to controlling rockmass displacement and deformation, and the optimum orientation is parallel to the maximum horizontal stress.