The invention discloses a precise positionable and compensable heavy-load mechanical arm which comprises a power device, a base, an arm assembly, a wrist assembly, a flexible claw and an automatic control system. The arm assembly comprises a lower arm assembly, a middle arm assembly and an upper arm assembly. The precise positionable and compensable heavy-load mechanical arm can adapt to a heavy-load working environment in a hydraulic drive mode, and on the basis, an external control system sends an instruction to an electric spindle according to information fed back by external and internal sensors and the like, so that a first connecting rod mechanism and a second connecting rod mechanism which are in transmission connection with the electric spindle perform fine adjustment compensation on the aspects of distance, angle and the like of the mechanical arm, and the precise effect can be achieved. When the mechanical arm is used, various sensor and computer control technologies are combined, so that working state monitoring can be achieved in the working process of the mechanical arm, precise mechanical arm operation can be ensured, meanwhile transmission errors of a transmission chain are decreased by using the electric spindle and other matched parts, the transmission accuracy is improved, and precise motion and safety guarantee of the wrist and the flexible claw is achieved.