Based on the theory of polarized light interference and multibeam interference, the transmission function of birefrigent Gires Tournois (BGT) interleaver is deduced. It is concluded that the higher the reflectivity is, the broader interleaver bandwidth and the lower contrast are whereas the narrower the bandwidth, the higher the contrast. Related experiments are designed and adjusted to verify the relationship between performance parameters of BGT interleaver. The structure parameters can be determined and tunable interleaver can be achieved by using the characteristics of BGT interleaver.