The extra-long optical cavity of optoelectronic oscillator (OEO) and the relationship between oscillation frequency shift and the variation of cavity length open the possibility for high-accuracy distance measurements at long range. Optical fiber not only affects the spectrum purity of the output microwave, but also determines the sensitivity of distance measurement. Based on the theory of optoelectronic oscillator and the distance measurement method, the effects of optical fiber length on spectral purity of OEO and measurement sensitivity are analyzed respectively. Fiber loops with different lengths are used in experiments in order to verify the results of analysis. Considering the demands of the measurement sensitivity, the spectral purity and the measurement range, as well as the error introduced by the environment, the fiber length of OEO will be about 1 km when the resolution of frequency measurement is 1 Hz.