A balance mechanism comprises a working device, a connecting arm assembly having a variable length, a torque-balancing assembly, a weighting member, and a connecting member. The connecting arm assembly couples to the working device. The connecting arm assembly is pivoted to the torque-balancing assembly by a pivot. The weighting member couples to the torque-balancing assembly. The connecting member connects the connecting arm assembly and the torque-balancing assembly. As the distance between the working device and the pivot changes, the distance between the weighting member and the pivot changes accordingly, so that a dynamic balance can be achieved.