With the development of laser technology, rectangular laser pulses can get narrower rising and falling edge, and the time scale of the rising or falling edge can reach nanosecond. When the falling edge of a rectangular laser pulse is incident to a Fabry-Perot interferometer and the time scale is less than the single round-trip time of light traveling in the Fabry-Perot cavity, the fast degradation of incident light intensity causes the change of the reflected light intensity and energy. The reflected light intensity and energy vary with the time scale of the rectangular laser pulse for an invariant phase difference of the light propagating through the Fabry-Perot cavity. For a fixed time scale of the rectangular laser pulse, the intensity and energy of reflected light vary with the phase difference. In this paper, the temporal response of the reflected light intensity and energy of the Fabry-Perot interferometer is studied, and a high precision sensing scheme can be achieved. To improve the sensitivity of the reflected light energy to phase difference by optimizing the parameters of the rectangular laser pulse and the Fabry-Perot interferometer, we calculate the signal-to-noise of the reflected light energy versus the response time, reflectivity and phase difference of Fabry-Perot interferometer. Furthermore, we analyze absorption property of the Fabry-Perot interferometer to expand the field of application of this sensing scheme.