UHVDC is more and more widely used in power grid, and multi-HVDC is adopted to connect the sending and receiving power grids. When one HVDC fault occurs, emergency power support is realized by runup other HVDC. In the case of weak receiving power grid, when one HVDC needs runup greatly, commutation failure will occur if the traditional extinguishing angle control strategy is adopted on the inverting side. Therefore, the causes of commutation failure and the influence of AC system strength and runup amount are analyzed. Then, an improved extinguishing angle control strategy on the inverter side is proposed, which introduce extinction angle measure value into the controller and realize closed-loop control, so as to ensure commutation margin and avoid commutation failure. Then experimental verification is carried out on RTDS simulation system, and test results show that the proposed strategy can solve the commutation failure problem when runup HVDC with weak receiving end greatly.