The dynamic security region (DSR) of an electrical power system has received more and more attention in recent years for providing plenty of security information and good prospect in online application. This paper proposes a new analytical method for determining the practical DSR (PDSR) based on the characteristics of hyper planes (HP) that are used to approximate the boundary of PDSR. In this method, a practical dynamic security criterion is derived from transient energy analysis while the power perturbation analysis is applied to pre-fault, fault-on and post-fault three system stages respectively. The key feature of such method is to analytically approximate the boundary of PDSR by solving linear algebra equations. Examples on the New England Test System show that this method can significantly reduce computation burden without compromising accuracy.