Disclosed are a concrete distributing device and a control method, a control system and an electrical control system for composite motion of a boom thereof. The control method comprises the steps as follows: calculating the flow of a boom pump when an operating handle sends a command; according to the command sent by the operating handle, calculating theoretical demand flow of all actuating mechanisms of the boom and theoretical control signals of all electronic proportional control valves; comparing the total theoretical demand flow of the actuating mechanisms with the flow of the boom pump; providing the theoretical control signals to all the electronic proportional control valves when the flow of the boom pump is greater than or equal to the total theoretical demand flow of the actuating mechanisms, otherwise, providing actual control signals smaller than the theoretical control signals to all the electronic proportional control valves and reducing the movement speeds of all the actuating mechanisms proportionally. In this way, when undersaturation of system occurs to the composite motion of the boom, the movement speeds of all the actuating mechanisms are reduced proportionally independent of the load difference of the actuating mechanisms, thereby ensuring that the boom can move in a motion track expected by an operator.