The utility model discloses a single-cavity piston type pneumatic brake valve for a diesel engine brake system of a commercial vehicle. The single-cavity piston type pneumatic brake valve comprises a pedal assembly (1), a piston (2), a valve body (3) and a lower valve assembly (4), wherein a brake lamp harness switch (5) is arranged on the valve body (3), and is connected with a brake lamp of the commercial vehicle through a circuit. Through the adoption of the technical scheme, a brake lamp switch device is arranged at the position of a control air opening, and a sealing gasket is additionally arranged at the position of the control air opening of the valve body, so the reliability of a product is improved; the fault rate is reduced to 0.6 percent from 4.2 percent; and the foundation is laid for the batch assembly of various kinds of vehicles.