The invention discloses a cut-through type cophase power supply system for cascaded input-free and output-free transformers. The system is mainly composed of a power electronics conversion device with three phases connected in parallel after each being cascaded with one or more AC-DC-AC converters of single phase-single phase structures. The three phases of the cascaded power electronics conversion device are directly connected with a traction contact net after being connected in parallel. The single phase-single phase AC-DC-AC converters can bear high voltage levels through the multi-module cascaded structure; meanwhile, the voltage levels output by converters are improved; thus, input and output transformers can be omitted, power required for locomotive loads is borne by the three phases on average, and energy can flow in two directions. Due to the fact that amplitude values, phases and frequencies of the output voltage can be controlled, contact nets close to substations can be directly connected to form a cut-through traction power supply network. By means of the cut-through type cophase power supply system, the input and output transformers of the traction substations can be omitted, the defects that the transformers in the traction substations are large in size, heavy in weight, troublesome to overhaul and high in manufacturing cost are overcome, the capacity of the traction substations is easily improved, and the running cost of the substations is reduced.