In order to realize the on-line measurement of a birefringence fiber (BF) loop mirror (Bi-FLM) strain sensor, the theoretical wavelength-demodulation on-line measurements expression of a Bi-FLM strain sensor was deduced. Two different groups of four-adjacent-wave-valley wavelengths near 1550nm and 1310nm were chosen and substituted into the theoretical expression. Both the calculated strains were basically consistent with the given strains. The result shows that the absolute BF length can be calculated by the random four-adjacent-wave-valley wavelengths, the initial BF length, the initial BF birefringence, and the initial strain dependent birefringence coefficient. Then the strain can be calculated according to the absolute BF length after axial strain and the initial BF length. According to the relative positions of any four-adjacent-wave-valley wavelengths which contained strain information, the noises or external sensing can be distinguished when the interference spectrum changes. By this way, the noises can be eliminated, and the measurement accuracy can be improved. These results can provide guidance for realizing the on-line measurement and improving measurement accuracy of the strain, vibration and other sensors of Bi-FLM.