Abstract: The length measurement method based on Optoelectronic Oscillator (OEO) introduces a free optical space, containing the measured length, into the optical cavity of OEO. The measured length is derived from the oscillation frequency corresponding to the cavity length. By virtue of the high sensitivity between oscillation frequency and group delay of the loop, this method promises high accuracy absolute length measurement at long range. However, long optical fiber, which is the high Q component of the cavity, is too sensitive to ambient temperature, leading to serious deterioration in accuracy and stability. In this paper, the influence of temperature variation was theoretically analyzed. The relation among thermally -induced error, temperature variation and length of optical fiber was established and analyzed, and optimization method of parameter was proposed. Experiments on optical fiber with different length and thermally-induced error were carried out. Experimental results consist with the error model, providing important basis for error compensation.