Quasi-steady model of the HVDC system is usually used in the studies on Sub-Synchronous Oscillation（SSO）， even its suitability is still arguable. The sample-data model of HVDC converter can represent the dynamic response of switching circuits， however， the firing control scheme and phase-locked loop（PLL） are not modeled in details. An improved sample-data model of 12-pulse HVDC converter is presented. Based on the generally used equidistant firing control scheme and PLL control， a new model of HVDC system is presented. Under small disturbances， the output of firing controller will not act on the converter immediately because of the igniting delay. The process of igniting is considered in detail， such that the sample- data model is improved. The model is validated on the EPRI HVDC system by eigenvalue analysis and complex torque coefficient method. The studies show that the improved model is more accurate in studying the SSO of HVDC systems.