Abstract: It is time consuming for traditional multi-infeed short circuit ratios (MSCRs) calculation method, which can not meet the requirement of fast analysis and decision-making of power grid when searching controlled islanding surfaces. In this context, a fast MSCRs calculation method that is applicable for islanded sub-system with multiple infeed HVDC lines of AC/DC power grid is proposed. It is assumed that the impedance matrix of AC power grid, the rated voltage of commutation bus bar, the rated DC power and the candidate system islanding strategy are known, then branch addition method is used to correct partial elements of impedance matrix rapidly, and the MSCRs of islands with the receiving end of HVDC lines are calculated according to the definition of MSCRs. The simulation results of New England 10-machine-39-node system and the Guangdong power grid prove the feasibility of this method. Compared with the traditional method, the proposed method is faster and more efficient.