Conventional fringe counting method has difficulty in direction judgment, especially at the peak or valley of cosine signal. In order to overcome this problem, we present a double Fabry-Perot (F-P) interferometer using a ratio fringe counting approach. Since the value of tangent signal jumps at which phase is an integer multiple of π/2, we use two orthogonal cosine interference signal ratio to get the tangent signal. Meantime, by counting the number of pulse, we can not only measure the phase difference but also judge the direction change. This approach has no strict requirement for initial phase difference of two optic paths. It′s very feasible for manufacturing, and experimental results demonstrate the linearly dependent coefficient up to 0.9999, a good linearity to 0.306% and a displacement resolution to ±3 μm within a dynamic range from 0 to 1mm of the sensor system.