A precision forging technology of tooth forms of duplex straight toothed spur gears is characterized in that a forging technology and a closed forging die are used to manufacture all the tooth forms of two-stage step cylindrical gears of a duplex straight toothed spur gear; according to the forging technology, a round cake blank is used for one-time heating, all the tooth forms are forged and formed at a time, the forging die structure for precision die forging is of a closed impression structure, a tooth-form female die is of a big-end-up split structure, a die upper punch is a split tooth-form male die, a die lower punch is a tooth-form cushion type die and also serves as an unloading die, and the dies are guided and positioned through four sets of guiding pillars and guiding sleeves. The technological process of forging includes the steps that material is prepared and heated, forging is performed on a no-bottom-dead-centre screw press or a die forging press with the closed impression forging die, hot forged pieces are inspected and cooled, two end faces and inner holes are mechanically machined, gear products are subjected to cementation heat treatment, the gear products are finally inspected, and products are packaged and put in storage.