This paper studies the symmetric extension applying in true two-dimensional even symmetric biorthogonal wavelet transform. According to the characteristic of true 2-D wavelet filter banks and symmetric extension, deduce the formula of four sub-band after wavelet decomposition. These formulas summarize the original data of any start point and any length at multi-level decomposition, obtain the cycle and symmetric relation of the four sub-band. Finally, take a true 2-D 5/3 wavelet transform as an example to illustrate the practical application of specific methods.