The invention discloses a method for identifying a variable boundary cable force of a medium or long cable. The method comprises the following steps of: performing unit discreteness on the cable, dividing the cable into n units (n is in a range of 40 to 400), and calculating a unit
rigidity matrix [K]e and a
mass matrix [M]e of each unit according to formulas (1) to (2); then arranging rubber
cushion blocks on the cable to form external restraint, wherein the number of the rubber
cushion blocks is 2 to 6 and the mounting number of the rubber
cushion blocks is determined according to the length of the tested cable; and after the total
rigidity matrix [K] and the total
mass matrix [M] are determined, solving a characteristic value equation l[K]-
omega2[M]l=0 of natural vibration frequency of a cable structure by adopting an iteration method to obtain the cable force T. Under the condition that the cable force is not changed, if the boundary condition of the cable is changed, the frequency of the cable is changed, and the equation of the frequency of the cable and the cable force is changed along with the change of the boundary condition of the cable, so the condition that the frequency of the cable cannot be tested under complex environments is solved, and meanwhile the frequency of the cable is in an interval capable of being tested by the conventional instrument by regulating the boundary condition.