In this paper, a pressureless sintering process of copper nanoparticles based on the copper formate pyrolysis reaction is presented, aiming to solve the problems of the existing copper sintering technology in which copper is prone to oxidation and requires auxiliary pressure. Copper nanoparticles (Cu NPs) were pretreated by formic acid solution to generate dense copper formate films, which were finally formed into Cu-Cu joints after sintering. After the joint sintering quality test, it was proved that a formic acid reaction time of 10 min, PEG solvent, 0.048 mm sandpaper polishing of the substrate surface and a heating rate of 5 ℃/min were the optimal sintering conditions. Under these conditions, pressureless sintering of copper nanoparticles was achieved, and the shear strength of the prepared joints could reach 16.18 MPa with a low resistivity of 5.7×10-6 Ω/cm. After 200 hours of high-temperature aging test, the shear strength of the joints could still reach 9.38 MPa, which verified the reliability of the sintering process. Therefore, the process proposed in this paper provides a new idea for realizing reliable interconnections of third-generation semiconductor chips.