The fabricating difficulties of the broadband filter were theoretically analyzed and a broadband bandpass filter was realized by the form of coaxial cavity. The miniaturization of the filter was achieved by the principle of loading capacitance. Finally, the broadband filter with the center frequency of 4.24 GHz and bandwidth of 44.3% was designed through the simulation software CST. The results showed that the designed filter had good performances of voltage standing wave ratio and outofband rejection, which could well meet the requirements of the handheld radar device.