Abstract: At present, study of DC line protection is mainly dependent on numerical simulation method. However, the model of voltage source converter based high-voltage direct current（VSC-HVDC） grid involves a large number of electrical nodes, hence the numerical simulation is extremely time-consuming and laborious. Furthermore, it is hard to propose an effective setting method for protections. In order to solve this problem, this paper developed a reduced model of VSC-HVDC grid for fault transient analysis. The complex frequency domain expressions of fault electrical quantities were derived for typical fault conditions and the inverse Laplace transformation were adopted to obtain the time-domain expressions. Utilizing the analytical results, the threshold setting method and sensitivity verification method of traveling wave protection were proposed. And the fault resistance endurance was calculated with different system parameters.Compared with numerical simulation method, the analytical solution can not only improve the calculation efficiency, but also obtain the variation ranges of the fault electrical quantities,which can effectively avoid the influence of the sampling data uncertainty to the protection performance. Moreover, the influence of the system parameters on the protection performance can also be analyzed conveniently, hence providing reference and advice for the selection of the parameters in practical situations.