Experiment on multi-mode fibre used for light transmiting,sinusodial phase modulating F-P interferometry has been performed.It has been proved by using Fourier analysis of intensity signal that either fundermental frequence amplitude or phase of intensity imply some information about the change of cavity length (or optical path difference).Therefore a method of time duration measurement for measuring phase or fundermental frequence amplitude has been suggested. Analyses of the sensitivity thresholds measured with the present set-up show that the method and the set-up could be free of temperature drift and simplify the signal processing, hence be more suitable for the on-line measurement.