Multi-terminal high-voltage dc (HVDC) grids are expected to experience a continuous expansion in the near future, and have appeared as competitive strategies in transmitting offshore wind and interconnecting multiple ac areas. This paper proposes the inertia sharing concept for the operation of multi-terminal HVDC (MTDC) grids, which can be achieved by the proposed Unified Reference Controller. The control objectives of the VSC stations are no longer limited to the stabilization of dc grid, instead, the requirements in ac side are also met. The interaction dynamics between the ac and dc grid is analyzed for illustrating the proposed concept. In addition, the VSC stations can work in different operation modes based on the unified control architecture, and further smoothly switch the operation modes for flexible maneuver. Simulation results exhibit good performance of the proposed strategy.