In this paper, six specimens made by four kinds of fiber materials used in sea cage were selected, the percentage of elongation of them were measured by two methods. The two forms regression equation of percentage of elongation and relatively load rate were established, one of them expressed the power function, and another expressed the linear function. The constant load elongation of new ropes and simulated loaded ropes, and the method to estimate at breaking elongation were brought forward. The attenuation of extensibility of rope during load and unload test according to 50% rated breaking load of it was discussed. The result of test and study on the elongation performance of the fiber ropes show that it may be the reference for the design, to calculate the length of the ropes used in the sea cage as traction cordage or anchor hawser.