This paper presents an optimization method of wideband filtering power amplifier (PA) with high efficiency. In order to achieve the expected high efficiency in the wide passband and reduce design difficulty, a high-performance area-based impedance target for computer-aided automatic optimization is proposed, which avoids the unexpected efficiency degradation caused by the conventional impedance target. In the meanwhile, the out-of-band optimization target described by impedance is also proposed to obtain bandpass filtering characteristic. The in-band and out-of-band optimization targets are utilized to design the output matching network to realize fundamental and harmonic impedance matching precisely, as well as the filtering response. For demonstration, a filtering PA operating in 3.3-3.8 GHz is implemented and measured. The measured small-signal$S$-parameter exhibits flat gain in passband and good bandpass filtering response. The measured power-added efficiency (PAE) is larger than 62% within the entire working band and the highest PAE is 71.7%.