Short-term frequency stability (STFS) is becoming a great concern for a regional receiving-end power system such as the East China Power Grid (ECPG) mainly for two reasons: (i) the increasing percentage of power injected from multi-infeed (ultra-) high-voltage DCs (HVDCs) with the skyrocketed capacities; (ii) the replacement of traditional generation by the continuously growing inertia-less renewables. However, the existing emergency control is not adaptive enough to maintain STFS under varying operation conditions. In this study, a real-time optimised control is proposed to solve the issue more efficiently. It combines emergency power boosting (EPB) of HVDC and emergency demand response (EDR) in a coordinated way. The optimal control problem is formulated based on the online-updated models that reflect the current state of the system and the dynamic responses of multiple types of generators. By converting the non-linear constraint into a linear matrix inequality, the allocation of EPB and EDR can be optimised in a real-time and coordinated way. The performance of the proposed control and its advantages over the existing one are verified by simulation studies on the model of ECPG with low, normal and high penetrations of renewables.