Weak parts of a HVDC transmission system can be recognized using sensitivity analysis， which has been an important technique for system reliability assessment and enhancement. A new sensitivity analysis method for HVDC system is proposed in this paper. The least square method is used to model the relationship of HVDC reliability indices varying with the reliability parameters of components. It can be used to obtain the sensitivity formula of system reliability indices with respect to parameters of system components. The concept of sensitivity curve and its analytical expression are also proposed. Based on the sensitivity curve， sensitivity indices can be estimated rapidly when the reliability parameters of components are changed. A single 12-pulse HVDC transmission system is used to illustrate the proposed method. Case studies indicate that the method is feasible and effective.