The invention provides an arm length-changeable mechanical arm joint module suitable for a seat-loaded small mechanical arm. The mechanical arm joint module comprises a motor guide rail unit, a synchronous belt drive unit, a box supporting unit, a driving control unit and an external connecting unit, wherein the motor guide rail unit and the synchronous belt drive unit are positioned and supported by the box supporting unit, a motor connection wire is connected onto a driving controller through reasonable layout, and the driving control unit is in charge of signal acquisition and processing and control on a motor, the motor guide rail unit drives an extension rod to rotate through the rotation of a first direct drive motor, so as to drive an external flange mounted on the extension rod to rotate; under supporting provided by the motor guide rail unit, the synchronous belt drive unit can move linearly along the motor guide rail unit under the driving of a second direct drive motor, so as to drive the external flange mounted on the extension rod to extend and retract, thus finally completing rotation, extension and retraction movements of the joint module.