The invention relates to an oil-immersed coupling reactor and a coil assembly thereof. The coil assembly comprises a first coil and a second coil which sleeve an iron core side by side in the length direction of the iron core, the winding directions of the first coil and the second coil are the same, close ends of the two coils are mutually connected to form a wire inlet end, and opposite ends ofthe two coils are mutually connected to form a wire outlet end; when power is on, magnetic fluxes which are equal in size and opposite in direction are formed between the first coil and the second coil for coupling to reduce circuit voltage loss; and when any one branch connected with the two coils is short-circuited, the coil corresponding to the short-circuited branch generates a magnetic flux far greater than that of the other coil to limit the short-circuited current. Partial magnetic flux is counteracted through coupling, so that the consumption of voltage and energy of a line by the reactor is reduced, and when any branch connected with the coils is short-circuited, the magnetic flux far larger than that of the other coil can be generated through the corresponding coil, so that the short-circuit current in the line is limited, and the safety of the line is protected.