Abstract: The growth of power supply demands and the improvement of power system capacity have put forward a higher requirements for the transmission capacity and operation reliability of cables in power transmission and distribution network. It is of great significance to effectively use the dynamic capacity expansion potential of high-voltage direct-current (HVDC) cable lines for the optimal allocation of electric power resources. In this paper, the finite element analysis (FEA) method is used to establish the transient thermal and electric field models of HVDC cables laid in different environments, and the temperature distribution and insulation electric field characteristics of cables under typical dynamic loads are simulated and analyzed. The research reveals that, compared with the systems laid in air or buried in soil, the HVDC cable system laid in a tunnel has the thermal response time constant of several hundred hours, and thus has the highest dynamic capacity expansion capability. Taking the cable laid in a tunnel as a case study, we calculated the allowable maximum overload currents of it under different initial load rates and permitted emergency durations through an iterative inversion process based on the corresponding FEA model. In addition, we proposed an evaluation technique for the short-term dynamic expanded capacity of HVDC cable lines, and analyzed the transient electric and thermal characteristics in the overload duration. The results indicate that, for HVDC XLPE cables with high initial load rates, long emergency durations and high temperature dependencies of the insulation conductivity, significant electric field changes will take place during dynamic capacity expansion, and thus should be given specific attention. For bipolar DC cable lines in tunnels with an initial load rate less than 50%, any one pole can safely run with 150% rated current for 2 hours. The findings of this paper provide theoretical guidance for dynamic capacity expansion and optimization scheduling of HVDC cable lines.