To explain the mechanisms of non-Gaussian features of wind pressure of the chamfered corner square cylinder, the flow around a sharp corner square cylinder and a chamfered corner square cylinder was investigated by large eddy simulation (LES) method with various wind angles at a Reynolds number of 2.2×10⁴. The effect of corner modification on the non-Gaussian features of wind pressure was analyzed. Based on the instantaneous flow information, the influences of corner modification on extreme wind pressure were discussed and the corresponding flow mechanisms were studied. Results show that the non-Gaussian regions of the sharp corner square cylinder mainly occurred at the rear corner of the lateral surface and the leeward surface. There were no noticeable non-Gaussian features at shear layer reattachment regions (i.e., separation bubble regions). Chamfer modification could significantly decrease the non-Gaussian regions at the rear corner of the lateral surface and the leeward surface. Flow mechanisms for the occurrence of the extreme wind pressure could be divided into two types, i.e., attached vortex mechanism at rear corner of the lateral surface and Karman vortex mechanism at the leeward surface. Compared with sharp corner square cylinder, chamfer modification made the separated shear layer closer to the cylinder, produced a weaker Karman vortex, and caused weaker (or even the disappearance of) attached corner vortices, leading to a decrease of extreme wind pressure and smaller non-Gaussian regions.