In this paper, the discoloration of the solder after the vacuum sintering of the power chip is analyzed. The morphology and composition of the discoloration area were characterized and analyzed by the metallographic microscope, SEM, EDS and XPS, and the discoloration mechanism was studied. The analysis results show that the main components of the color-changing solder are C, O, Pb, Sn, Ni, Ag, and Au, which are the normal components of the solder after vacuum sintering. The reason for color-changing is the oxidation of the Sn element, and the different thickness of oxide shows different colors. The discoloration solder was treated with formic acid, and the solder returned to bright silver-white. The test further confirmed that the discoloration was caused by metal oxidation. The problem of solder discoloration is solved by optimizing the process to reduce the sintering time and vacuum defoaming time.