Travelling-wave Protection (TWP) is the main protection of HVDC transmission lines, and it is important to act correctly for TWP to ensure the system safety when a DC-line-short-circuit fault occurs. However, since TWP uses the transient electrical quantities in the initial fault stage to construct its action criteria, the results of internal-fault discriminant from a practical TWP scheme are highly uncertain, or probabilistic. To quantitatively describe this probability, this paper proposes an action probability calculation and analysis method for TWP against DC-line- short-circuit faults. Based on the actual TWP operation logic and different system operation modes, this method considers four uncertain fault condition factors, including fault type, fault point, fault resistance and fault time, and adopts Latin hypercube sampling technique to calculate related probability indexes. In the numerical examples, the actual TWP scheme in YunGuang ±800 kV UHVDC system is taken as the study object, and the influences of different fault condition factors and the changed operation modes to TWP are analyzed. This work is supported by China Postdoctoral Science Foundation 60th Surface Project (No. 2016M602436).