The invention discloses a balancing device allowing big arm supporting force of a mechanical arm to be adjusted and a parameter optimizing design method thereof. According to the device, a pneumatic supporting cylinder is arranged between a big arm of the mechanical arm and a big arm base. The two ends of the pneumatic supporting cylinder are hinged to the big arm of the mechanical arm and the big arm base respectively. A pneumatic pressure control loop is connected between the pneumatic supporting cylinder and an air source loop. The pneumatic pressure control loop is formed by connecting an air source, a pressure-stabilizing type pressure reduction valve, a tiny mist separator, an electric proportional pressure valve, a piloted pressure reducing valve, a throttling valve and a pressure meter. By means of the pneumatic pressure control loop, air supply pressure of the pneumatic supporting cylinder can be adjusted in a stepless mode in real time, the pneumatic supporting cylinder can properly output balance supporting force needed by the big arm according to loads, and therefore the big arm can carry the heavy loads smoothly with small drive moment.