A compensation method for a position error caused by a length change of the tail pole of a decoupling six-degree-of-freedom mechanism is provided. The method establishes a position error compensation matrix, obtains a compensation according to the actual length of the tail pole and the position error compensation matrix in a test using the decoupling six-degree-of-freedom mechanism, compensates an error caused by the length change of the tail pole of the decoupling six-degree-of-freedom mechanism according to the compensation, not only makes the compensation result more accurate, but also achieves a simple compensation process, fast speed and high efficiency, and can fast compensate the coupling error caused by the length change of the tail pole of the decoupling six-degree-of-freedom mechanism.