A supporting arm assembly has an upper arm, a lower arm, a display-connecting device and a holding device. The lower arm is pivotally connected to the upper arm. The display-connecting device is mounted on the upper arm and has a pivotal pin, a connecting base, a display mount, an engaging panel and a display holding panel. The pivotal pin is mounted rotatably on the upper arm. The connecting base is connected pivotally to the pivotal pin. The display mount is mounted securely on the connecting base. The engaging panel is mounted in the display mount. The display holding panel is mounted detachably in the display mount and detachably engages the engaging panel by an engagement between at least one engaging protrusion and at least one engaging recess. The holding base is mounted on the lower arm at an end opposite to the upper arm.