Pantograph-catenary (PC) arc has become one of important factors which effect the behaviors of pantograph-catenary system. Understanding on the underlying physical mechanisms of PC arc dynamics is still far away from satisfactory, therefore researches from both experimental and numerical aspects are urgently required. This paper focuses on the dynamic characteristics of pantograph-catenary arc during the pantograph lowering process by numerical simulation method. A pantograph-catenary arc model is established based on the classical magneto-hydrodynamics (MHD) theory, with the temporal evolution of PC arc dynamics taken into account. The dynamic mesh technique provided by FLUENT is adopted to take care of the departure process between the pantograph and catenary. Detailed behaviors of arc plasmas during the pantograph lowering process are presented by the temperature distribution and flow filed. Two interesting vortexes located below the catenary are found. The vortexes rotate in the opposite direction, resulting from the special gas flow motion during the pantograph movement. Furthermore, the vortexes bring about a thermal pinch effect on arc column, which has a significant influence on the next behaviors of arc plasmas. The arc characteristics under different pantograph lowering speeds are analyzed. The characteristics of pantograph-catenary arc are influenced more obviously under the action of gas flow in the condition of higher pantograph lowering speed.