The invention discloses a system having a modular mechanical DC breaker. The system comprises an energy absorption branch, a current carrying branch and a commutating branch, wherein the energy absorption branch, the current carrying branch and the commutating branch are connected in parallel, the energy absorption branch is used for absorbing residual energy, the current carrying branch is composed of N low voltage modules connected in series, the N is not smaller than 1, the low voltage modules comprise mechanical switches, the current carrying branch is used for controlling closing of the mechanical switches when the system is in normal operation, and the commutating branch is further used for realizing arc blowout opening when the system is in fault. The system is advantaged in that according to different system voltage levels, the low voltage modules in different quantities can be flexibly selected for series connection, the system is easy to expand to the high voltage level, moreover, the opening principle is reliable, and reliable opening of the higher voltage level and the more heavy fault current can be realized.