The invention discloses a mating gear machining method for a formed gear, and belongs to the technical field of mechanical manufacturing engineering. The mating gear machining method involves a matinggear machining process route and a modification coefficient allocation and calculation software package with the gear pair sliding rate difference delta eta smaller than or equal to 0.0001. Accordingto selection of the modification coefficient of a mating pinion, a big gear actual modification coefficient x2' calculated according to the actually measured common normal length average value of a machined big gear is substituted into the modification coefficient allocation and calculation software package with the gear pair sliding rate difference delta eta smaller than or equal to 0.0001, a mating pinion modification coefficient target value x1' and a pinion target common normal length value W1k' are calculated. A mating pinion is machined according to the mating pinion modification coefficient target value x1' and other known design parameters, precision manufacturing with the gear sliding rate difference close to zero is achieved, the mating gear machining method is mainly applied tomanufacturing the low-speed heavy-duty mating gear, the service life of the gear can be prolonged, and the transmission power consumption can be reduced.