  Ramsey spectroscopy via coherent population trapping (CPT) is essential in
precision measurements. The conventional CPT-Ramsey fringes contain numbers of
almost identical oscillations and so that it is difficult to identify the
central fringe. Here, we experimentally demonstrate a temporal spinwave
Fabry-P\'{e}rot interferometry via double-$\Lambda$ CPT of laser-cooled
$^{87}$Rb atoms. Due to the constructive interference of temporal spinwaves,
the transmission spectrum appears as a comb of equidistant peaks in frequency
domain and thus the central Ramsey fringe can be easily identified. From the
optical Bloch equations for our five-level double-$\Lambda$ system, the
transmission spectrum is analytically explained by the Fabry-P\'{e}rot
interferometry of temporal spinwaves. Due to small amplitude difference between
the two Land\'{e} factors, each peak splits into two when the external magnetic
field is not too weak. This peak splitting can be employed to measure an
unknown magnetic field without involving magneto-sensitive transitions.
