The present invention is to provide a narrow bandpass filter installed on a circuit board for suppressing a high-frequency harmonic wave, which uses a metal microstrip method to directly build a microstrip circuit corresponding to a narrow bandpass filter on a circuit board of a wireless communication product and uses an interdigital finger method to lay a capacitor microstrip of the microstrip circuit on the circuit board to avoid the capacitor microstrip occupying too much area of the circuit board and maximizes the usable space of the circuit board, so that the wireless communication product can suppress a second or higher harmonic spurious noise of the wireless communication product without using an additional high-frequency filter, and thus can greatly lower the manufacturing cost of the wireless communication product and effectively reduce the electromagnetic interference.