Dual Mechanism for optical chirped pulse being compressed with a F P resonator always simultaneously exists, which is the mechanism of filtering effect and dispersion effect. Dispersion of F P resonator can change the phase of wave spectrum of chirped pulse and appear as normal or extraordinary characters, depending on the difference between the tuning wavelength of F P resonator and the optical carrier wavelength. It turns out that the optimal tuning wavelength range of F P resonator for compression of chirped pulse is a set of noncontiguous wavelength regions separated form each other with spacing of about several nm. The results here also have successfully explained the experimental results by M. Nakazawa et al in 1990, for the first time.