The invention discloses a restoration method for a disqualified gear engagement contact spot in the speed reducer assembling process, and belongs to the technical field of speed reducer assembling. The method includes the following steps that 1, a driving gear tooth face and a driven gear tooth face are engaged, thin film paint is sprayed to a driving gear, and thin film paint is sprayed to part of the driven gear tooth face; 2, a white sticker adheres to the engagement-free contact spot of the driving gear tooth face and the driven gear tooth face, and then engagement dyeing is conducted until an involute of the paper completely covers the contact spot; 3, the sticker is taken down, a micrometer is used for measuring the thickness of the sticker, restoration amount and helical angle change data are calculated according to the thickness, and data are input into a forming grinder for tooth face grinding. The parameter change amount of the tooth thickness reduction amount and the helical angle can be accurately calculated only through a simple test, calculation and operation are simple, restoration cost and labor cost are reduced, and the restoration method is suitable for application and popularization in production operation, and has high practicality.