The detection and isolation of DC line faults is an urgent problem to be solved in flexible DC grid. In this paper, based on the basic structure of flexible DC grid, by analyzing the DC line fault protection requirements for DC line protection, a transient-based protection based on boundary principle is introduced, and applicability analysis is performed. It analyzes the necessity of quickly tripping the DC circuit breakers at both ends of the DC line when a DC line fault occurs. Then on the basis of improving the measurement sampling frequency and the optimization criterion time window and according to the different characteristics of the fault reverse wave energy and the forward wave energy at the power delivery converter station and the receiver station, it optimizes transient-based protection criterion and provides a protection scheme for the both ends of the DC line. On this basis, it further researches the fault clearing strategy of DC link transient-based protection cooperation with DC circuit breaker and DC bus protection. The feasibility of the proposed strategy is verified by simulation. This work is supported byKey Project of Smart Grid Technology and Equipment of National Key Research and Development Program of China (No. 2016YFB0900600) and Science and Technology Project of State Grid Corporation of China (No. 52094017000W).