On the basis of the principle of probability preservation and its random event description, a new kind of probability density evolution method (PDEM) has been developed by Li and Chen at the beginning of this century. The existing investigations demonstrate that the generalized probability density evolution equation (GDEE) reveals the secret of randomness propagating in a physical system: the transition of probability density of a stochastic system definitely relies on the change of physical state of the system. By virtue of these developments, a series of physically-based studies on system reliability have received extensive attention, and therefore supply a unified perspective for engineering reliability analysis of structures and lifeline networks. This paper briefly describes the theoretical foundation of probability density evolution method in view of a broad background, and a general framework for engineering reliability analysis, termed as the Physically-based Comprehensive Method (PCM), is proposed. For illustrative purposes, two cases are addressed to prove the value of the proposed method.