Sn2.5Ag0.7Cu0.1RE0.05Ni lead-free solder alloy was used as the research object. Based on the unique structure, excellent physical properties, and good mechanical properties of graphene nanosheets (GNSs), the Ni modified GNSs (Ni-GNSs) were used as the reinforcement phase. The soldering process of Ni-GNSs reinforced SnAgCuRE system composite solder/Cu and thermal aging tests of soldering joints were conducted to investigate the effect of Ni-GNSs on the microstructure and thermal aging fracture mechanism of composite soldering joints. Results show that the addition of Ni-GNSs inhibits the linear expansion of the composite solder, resulting in lattice distortion and dislocation. The intermetallic compound (IMC) particles near the dislocation line interact with the dislocations and hinder their movement, thereby strengthening the composite solder and further improving the soldering joint. With a longer thermal aging time, the thickness of interface IMC layer is increased and the shear strength of soldering joints is decreased. Among them, the shear strength decrement of the composite soldering joints with 0.05wt% GNS addition is the least of only 8.9%. Moreover, after thermal aging for 384 h, its shear strength is still higher than that of the Sn2.5Ag0.7Cu0.1RE0.05Ni/Cu soldering joint before thermal aging. With the addition of Ni-GNSs, the growth coefficient of interface IMC of composite soldering joints is significantly reduced, which effectively alleviates the degradation of mechanical properties of composite solder/Cu soldering joints during the thermal aging process, further changes the thermal aging fracture mechanism of composite solder/Cu soldering joints, and ultimately affects the reliability of joints. The fracture position of the Sn2.5Ag0.7Cu0.1RE0.05Ni/Cu soldering joints moves from the soldering seam before thermal aging to the soldering seam/interface IMC, presenting the ductile-brittle mixed fracture. The fracture position of the Sn2.5Ag0.7Cu0.1RE0.05Ni-0.05GNSs/Cu soldering joints is still in the soldering seam zone, presenting the ductile fracture, which indicates the high reliability of the soldering joints.