Lyocell fibers was treated by caustic treatment and cellulose treatment, and we observed sorptcon isotherm and behaviour of different molecular mass of direct dyes on lyocell fibers. We find that lyocell fibers of aborption was affected by caustic treatment, cellulose treatment, the suqeuncy of treatment and the character of absorbate. We find exhaustion that lyocll was treated by cellulose treatment, then treated by caustic treatment is better than others. And we find 5% caustic treatment is better than other density.