We use femosecond laser pulse to study the fringe variation on the spectrum of pulse due to F-P effect in PPQ organic polymer film. By changing the pulse peak intensity, the fringe variation is observed evidently, while fringe variation is not observed at broadened pulse duration and the same average power. It shows that the thermal effect is no greater contribution to the refractivity, the main reason that leads to fringe variation on the spectrum can be attrivuted to the nonlinear refractive index change of PPQ film due to high pulse peak intensity.