A coaxial mechanical automatic gear selecting and shifting system used for an automobile and driven by two servo motors comprises a first servo motor, a second servo motor and a mandrel. An internal thread is prefabricated on a rotor of the first servo motor and is in matched connection with a screw portion at one end of the mandrel, an internal spline is prefabricated on a rotor of the second servo motor and is in matched connection with a spline portion at the other end of the mandrel, more than two gear shifting blocks are uniformly distributed on an optical axis portion in the middle of the mandrel, each gear shifting block is matched with one gear shifting unit which comprises a shifting fork, a shifting fork big hole is connected with a supporting sleeve sleeving the mandrel through a sliding bearing, an inserting block is fixed on each shifting fork by a cover plate through a screw, each shifting fork is in linear motion under supporting and constraining of the supporting sleeve, and each inserting block is matched with the gear shifting block. A screw-nut structure is integrated inside each motor, so that space is saved; the mandrel and the inserting blocks are utilized to complete gear selecting and shifting actions, so that structure is simplified; the servo motors are used as a power source, so that control accuracy and reliability are improved.