Disclosed are a method for integrating a single-axle parallel-connection hybrid system and a device. A speed changer is fixedly connected with a motor in a spigot positioning manner, the motor is fixedly connected with a flywheel shell of an engine in a spigot positioning manner, and an input shaft of the speed changer is connected with an output shaft of the motor via splines; and a blind hole inner spline is arranged at the end of the input shaft of the speed changer, an outer spline which matches with the inner spline at the end of the input shaft of the speed changer is arranged at the end of the output shaft of the motor, the outer spline of the output shaft of the motor is directly inserted in the blind hole inner spline at the end of the input shaft of the speed changer, and accordingly the input shaft of the speed changer is connected with the output shaft of the motor via the splines. In addition, a motor shell and a clutch shell share the same side wall, an original side wall of the clutch shell is omitted, and a support of a clutch operating lever moves on the side wall of the motor shell. The splines are involute splines, the involute splines are modified, and axial pressure and axial force of the splines due to errors of coaxiality are weakened.