Aiming at the requirement of flat bandpass filter in modern wireless communication system, a hairpin resonator with parallel load is proposed, and a fourth-order flat bandpass filter is constructed by using the proposed resonator. Firstly, a new parallel resonator is designed by setting a resistor at the strongest magnetic field (central position) of the hairpin resonator, so that the inherent quality factor can be flexibly adjusted by changing the resistance value; Secondly, the fourth-order flat bandpass filter is constructed by using the coupling topology with high and low intrinsic quality factors. Finally, a fourth-order bandpass filter with transmission zero and flat bandpass is fabricated on the printed circuit board. Experimental results show that the measured results of the fourth-order bandpass filter are in good agreement with the simulation results. Compared with other multi-order bandpass filters, the fourth-order bandpass filter has the inherent quality factor that can be flexibly controlled and the passband flatness is less than 0.2dB.