The boundary of dynamic security region（DSR） can be approximated as one or a few hyper planes（HPs）. Based on this property， a fast method to compute DSR is developed. The method performs active power perturbation analysis respectively on the stable equilibrium point of pre-fault system， the trajectory of fault-on system and transient stability of post-fault system. Then， according to the continuities of state variables during transient responded process， results from different stages are connected one by one and the HP expression of DSR is finally obtained. In such method， a practical dynamic security criterion in power injection space， derived using the concept of transient energy function， is deployed to quantify transient stability and implement post-fault power perturbation analysis. Examples on the New England Test System show that the proposed method can significantly reduce computation burden without compromising accuracy.