Abstract: From the view of the coding gain，this paper investigates the performance of 2-channel biorthogonal wavelet image copression system.Firstly，the polyphase matrix of biorthogonal wavelet filter bank is decomposed by lifting scheme，then lifting parameters with one variable are obtained combining the vanishing moment and normalization conditions.According to the Vaidy- anathan’s coding gain，the optimal value can be got.In the example，three 9-7 wavelets are constructed significantly.Although MIT wavelet is slightly lower than CDF 9-7 on the quality of reconstructed image，it is friendly implemented in hardware.Furthermore，its coding gain is more than the other two wavelets.This should be given consideration in the design of coder.