A single bridge arm water-cooling current transformer device comprises a box body, an IGBT module with a water-cooling radiator, AC busbars, a low-inductance laminated busbar formed by the lamination of a positive DC busbar and a negative DC busbar, a grading resistor, a DC bus capacitor, an absorption non-inductive capacitor and a cooling fan, wherein one side edge of the positive DC busbar and one side edge of the negative DC busbar are folded inwardly, so as to form a radiating air passage of the DC bus capacitor with a module side mounting plate; the cooling fan is mounted at the top of the box body above the radiating air passage; the anode and the cathode of the DC bus capacitor are electrically connected with the positive DC busbar and the negative DC busbar through bolts respectively; and the grading resistor is mounted on the outer side surface of the low-inductance laminated busbar through the bolts. The device has the advantages that the cooling manners of power components and the DC bus capacitor adopt water circulation and forced air cooling respectively, thereby avoiding mutual interference; the elimination of cables and wires improves the reliability; and a casing of the module is made of copper and reliably grounded, thereby improving the safety and shielding the electromagnetic radiation and interfere caused by the IGBT turn-off due to rapid conduction.