The invention provides a joint structure of a modular mechanical arm. The end face of the upper part of a supporting framework of the joint structure is formed as an arch-shaped connection region; theend face of the lower part is formed as a rectangular fixation region; the middle part is provided with a driver; an output shaft, positioned at a linkage side of the supporting framework, of the driver is provided with a driving wheel; the other side, opposite to the driving wheel, of the supporting framework is formed as a driving side; the upper part, close to the connection region, of the supporting framework is provided with a speed reducer; an input shaft, at the same side with the driving wheel, of the speed reducer is provided with a linkage wheel; the speed reducer at the driving side is provided with a speed reducer output shaft; one side, close to the speed reducer output shaft, of the speed reducer is provided with a mounting plate; the driving wheel and the linkage wheel arein transmission connection through a closed belt; the lower part of the supporting framework is provided with a joint integration controller; the joint integration controller is connected with the driver; and the part, close to the edge of one side of the supporting framework, of the fixation region is provided with a connector so as to solve the defects that an existing mechanical arm is not precise in rotation, inconvenient to maintain and tedious and complicated in control.