In this paper, a third order differential bandpass filter with adjustable center frequency is presented. The filter uses the structure mainly consisting of a folded stub loaded resonator (FSLR) and a pair of folded parallel coupled lines to achieve filtering characteristics, at the same time, the microstrip-slotline conversion structure is used to excite the differential mode and suppress the common mode. Compared with the traditional microstrip filter, the common mode response of the proposed structure is independent of the differential mode response, so the good common mode suppression can be realized without reducing the differential mode performance, which makes the design more flexible. The overall size of the differential filter is 26 mm×49.2 mm, the center frequency is 3 GHz, and fractional bandwidth(FBW) is up to 28.67%, the level of common mode suppression is better than 37 dB. The results show that the measured results are in good agreement with the simulation ones.