Graphene aerogel (GA) has wide applications in energy storage, catalysis, and adsorption. However, as a candidate for supercapacitor electrode, it suffers from low specific capacitance. In this paper, we conceived and conducted a two-step strategy to improve the electrochemical properties of GA: First, GA was selective functionalized with p-phenylenediamine. Then PANI was grafted onto GA by in situ polymerization. The results suggest that surface functionalization enhances the interface bonding between GA and PANI and charge transfer during electrode redox. As a result, the obtained GA(p)/PANI composites, applied as the freestanding electrodes for supercapacitor, exhibit high specific capacitance (810 F/g at 1 A/g, 2.4 times of GA), good rate capability, and extraordinary cycling stability (83.2% of retention at 10 A/g after 10,000 cycles, 1.25 times of PANI/GA), indicating its great potential in the construction of high-performance energy-storage systems.