The invention discloses a control method, a control system and an electrical control system of composite motion of a cantilever crane of concrete distributing equipment, wherein the cantilever crane is a load-sensitive hydraulic system with pressure compensation. The control method comprises the steps of: according to a command sent by an operating handle, calculating theoretical demand flow of all actuating mechanisms of the cantilever crane, theoretical control signals of all electric proportional control valves and flow of a cantilever crane pump when the operating handle sends the command; when the flow of the cantilever crane pump is saturated, providing the theoretical control signals for all the electric proportional control valves; otherwise, providing real control signals smaller than the theoretical signals for all the electric proportional control valves and reducing the motion speeds of all the actuating mechanisms in the same proportion. According to the invention, when undersaturation of system flow occurs to the composite motion of the cantilever crane, the motion speeds of all the actuating mechanisms are reduced in the same proportion without any relation with the load difference of the actuating mechanisms, thereby ensuring that the cantilever crane can move in a motion curve expected by an operator.