The invention belongs to the technical field of intelligent assembly and particularly relates to an automatic installing device for a washer and a nut and a washer and nut installation method. The automatic installing device comprises a washer feed vibrating disk, a nut feed vibrating disk, a washer and nut distributing mechanism, a washer and nut fetching mechanical arm, a nut screwing transversemovement mechanism and a bolt fixing mechanism. The washer and nut distributing mechanism comprises a first lifting cylinder, a discharging plate, a feed cylinder, a nut feeding plate, a washer feeding plate, a nut centering cylinder, a nut centering plate, a washer centering cylinder and a washer centering plate. The nut screwing transverse mechanism comprises a transverse guide rail, a first slider, a rotating cylinder, a first motor support, a nut screwing motor and a nut and washer holding head. According to the automatic installing device for the washer and the nut and the washer and nutinstallation method, the washer and nut distributing mechanism stacks the washer and the nut, the washer and the nut are moved to a bolt on the bolt fixing mechanism through the nut screwing transverse movement mechanism, the nut is screwed on the bolt, and full-automatic screwing of the washer-nut assembly is achieved.