The invention discloses a duplex gear speed reducing device. The duplex gear speed reducing device comprises a shell, a first-stage inner gear ring, a planetary plate, a duplex gear, a second-stage inner gear ring and a connection gear; the first-stage inner gear ring is fixed to the shell; an end cover is arranged on the end portion of the shell; the planetary plate is installed on an output shaft of a hydraulic motor through a bearing; the duplex gear is installed on the planetary plate and comprises a large-diameter gear and a small-diameter gear; the connection gear is fixed to a shell of the hydraulic motor; the second-stage inner gear ring is arranged on the connection gear in a sleeving manner and is engaged with the connection gear; the large-diameter gear of the duplex gear is engaged with driving teeth on the output shaft of the hydraulic motor and the first-stage inner gear ring; and the small-diameter gear of the duplex gear is engaged with the second-stage inner gear ring. The duplex gear speed reducing device is compact in structure, small in size, small in weight, steady in movement and long in service life.