Objective To determine the water absorption coefficient of single-flavor root and rhizome Chinese herbal medicine pieces at room temperature, and guide the water addition in the decoction process of decocting machine of Chinese herbal.Methods The water absorption coefficient of 222-flavor root and rhizome Chinese herbal medicine pieces were studied, the simulated prescriptions were decocted according to the recommended formula of the decocting machine manufacturer and the water absorption coefficient, and the amount of liquid were obtained by the two methods which were compared with the amount of liquid required.Results The water absorption coefficients of roots and rhizomes with different textures were quite different.The amount of liquid obtained according to the manufacturer′s recommended formula was quite different from the amount of liquid required and there was no rule to follow.The error of the amount of liquid obtained according to the water absorption coefficient and the amount of liquid required was small and regular.Conclusion The experimental determination of the water absorption coefficient of traditional Chinese medicine decoction pieces could guide the amount of water added to the decoction machine.