Abstract: Flexible DC distribution system is an important technical mean to connect renewable energy and promote the development of distributed energy. Its overcurrent calculation is the basis of current limiting scheme design, equipment selection,protection strategy, etc. The accurate theoretical calculation method with universality can simplify and guide a large number of electromagnetic transient simulations of overcurrent. Therefore, a systematic analysis on the error causes, influencing factors and application scopes of the typical theoretical calculation methods of overcurrent in radial and circular flexible DC distribution systems under pole-to-pole short circuit faults is carried out in this paper. Firstly, the analytical calculation method of overcurrent in radial flexible DC distribution systems and the numerical calculation method of overcurrent in circular flexible DC distribution systems are analyzed, respectively. It is proposed that the fundamental causes of errors in theoretical calculation methods are line model errors and public line coupling. Then, with different topological structures, the influence of current limiting reactors, line type and line length on the theoretical calculation error of overcurrent is analyzed. Finally, for different operation conditions, the applicability of the theoretical calculation methods for overcurrent in flexible DC distribution systems with different topologies is proposed.