Accurate short-circuit fault current calculation is important for device selection,parameter design and protection research of flexible DC grid. To solve the problem that existing fault current calculation methods do not consider the pole-to-pole coupling of DC lines,a short-circuit fault current calculation method based on phase-mode transformation is proposed. First,the equivalent fault models of the converter station and DC line are established. Then,the phase-mode transformation is used to achieve electrical decoulping of positive and negative poles,and the node impedance matrix of the DC grid in the mode-domain is obtained. Subsequently,combining the fault boundary conditions of the DC grid,the fault current of the DC grid is calculated in the complex frequency-domain,and the fault current in the time-domain is obtained by using vector fitting. Finally,the proposed fault current calculation method is verified by PSCAD/EMTDC. The simulation results show that the proposed method can accurately calculate the DC fault currents after the DC fault occurs and before the converter station blocks.