This article describes a methodology for designing broadband high-efficiency GaN HEMT power amplifier (PA) which is suitable for base stations and modern wireless communication based on the Class-EF mode. To achieve both efficiency enhancement and bandwidth extension for the PA, the load-pull technique is employed to acquire the optimum impedance regions of the second harmonic. Especially, considering the influences of the harmonic control network on the accuracy of obtaining the fundamental impedance, the load-pull technique is performed to obtain the optimal fundamental impedance again after the harmonic control circuit is determined. In addition, the theory of multiple frequencies matching is also employed to manipulate six desired fundamental frequency points concurrently. Based on the proposed theory, the Class-EF PA prototype is designed and fabricated. The measurement results exhibits a drain efficiency (DE) of 60%-75% in the range of 2.1-3.6 GHz while having an output power of 38-40.5 dBm and gain of larger than 8 dB.