An integrated method of broadband duplex-filtenna based on a 3-D metallic cavity structure is proposed in this paper. In our design, two broadband filtennas with different operation frequency bands share the same metallic cavity-backed resonator. The two operation bands are realized with fractional bandwidths (FBWs) of 24% and 18%. The whole total FBW of two bands achieves 57%, and there are radiated nulls at the transition band showing a good filtering property. By integrating these two filtennas, a duplex-filtenna is formed with unidirectional radiation property. The radiation gain of these two filtennas exhibits good filtering property. The filtenna gain frequency responses have two transmission zeros near the lower and upper cutoff frequencies of both lower and upper operation bands. To validate the proposed concept, one cavity-backed duplex-filtenna with low-profile metallic cavity is designed and fabricated. The measured results have verified the simulation results well, and the isolation between the two wideband channels exceeds 30 dB as predicted in theory.