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A wide stopband and high isolation balanced diplexer (BD) using asymmetric hairpin ring resonator (A-HRR) is presented in this brief. By virtue of the flexibility of the A-HRR, the removed differential-mode (DM) spurious harmonics and common-mode (CM) frequencies of the A-HRR are analytically illustrated. And the A-HRR is applied to the design of the proposed BD. Due to the complexity and inaccuracy of the conventional design method of the BD, the grey wolf algorithm (GWA) is adopted to optimize the physical parameters of the BD circuit to reach the desired performance rapidly and accurately. Finally, the BD operating at 3.4/4.4GHz is fabricated and measured for verification. The measured DM isolation is 51.88/52.46 dB in-band, and it shows a stopband up to $4.55f_{1}$ with 15dB rejection.