The invention relates to a relay valve for a brake system of a railway vehicle. The relay valve comprises a valve body and a valve stem arranged inside a cavity of the valve body; three air channel connectors communicated with the internal cavity of the valve body are arranged on the valve body, two of the air channel connectors are air inlet connectors which are respectively connected with a brake air storage cylinder and controlled pressure, and the other air channel connector is an air outlet connector and is connected with a brake cylinder or a train pipe; an elastic recovery assembly is sleeved on the upper portion of the valve stem, the lower end of the valve stem is in contact connection with a valve port, and a central through hole communicated with the external atmosphere is arranged inside the valve stem; and a pressure regulating assembly placed below the valve stem is further arranged inside the valve body and is used for controlling break-make between the brake cylinder or the train pipe and the atmosphere or the brake air storage cylinder. The relay valve is simple in structure, low in cost and fast in response, and has fast air charging and exhausting properties, outputted pressure can be stably and linearly changed along with change of the controlled pressure, and can be controlled stably, the air flow can be increased or decreased according to the change of the controlled pressure, and functions of air charging, air exhausting or pressure maintaining are realized, so that braking force of the vehicle or a train is controlled.