In this paper, the release process of flexible optical fiber from the spool dispenser is studied. In general, the method of adding elements is used to solve the variable-length problem of the flexible optical fiber. However, when this method is applied to optic fiber release with large length, it will cause large computation and difficult convergence. In this paper, the method of power function polynomial fitting of optic fiber posture function and length normalization is used to establish the optic fiber release model. Galerkin method is used to simplify the partial differential equations into differential equations and solve them. to verify the correctness of the theoretical calculation, a test was carried out using an unmanned aerial vehicle carrying a miniature. According to the state of UAV, the shape, falling track and tension distribution of the optical fiber in the air are calculated. The test value of the ground end tension is compared with the theoretical calculation value. The change trend of the two is basically consistent, and the biggest difference is 12.7%. The reasonableness of the modeling and numerical solution method is proved by the above experiment, which provides a method for solving the variable length release problem of large length, low linear density, and slowly changing shape, performing good convergence. This method provides a reference for failure analysis, fault recurrence and trajectory optimization of guidance fiber.