Based on signal processing theory, the filtering characteristic of thin film filter was researched in detail. Thin film filter is minimum phase filter (MPF) and Hilbert transform law is related with the amplitude response and the phase response. So the phase response can be reconstructed from its amplitude response and then the characteristic of chromatic dispersion can be achieved. To validate the theoretical analysis, a test system for group velocity delay was set up based on RF modulation. As an example, the group velocity delay of a 100 GHz 33CH thin film filter was tested. The experimental result matched with the theoretical result very well through contrasting analysis. At last, an analysis of the characteristic of chromatic dispersion of thin film filter was also provided.