Based on theoretical research on the characteristics in transient processes， such as line faults， communication failure， lightning stroke etc.， which occur in a±800 kV UHVDC transmission system， this paper makes a multiple-dimensioned analysis of various transient voltages using wavelet transform. Analysis results show that the energy distribution of the fault component in the transient voltage reveals the characteristics of the corresponding transient process and the transient protection based on these characteristics can reliably identify DC line faults. Comprehensive simulation using PSCAD/EMTDC has been made that validates the high reliability and speed of the approach presented， which can satisfy the requirement on UHVDC transmission line protection.