OBJECTIVE：To establish mathematical model of liquid volume added of TCM medicinal broth decoction machine to accurately calculate liquid volume added in the process of medicinal herb decocting，so as to guarantee the quality of medicinal herb decocting. METHODS：The water absorption rate of representative TCM decoction piece with high use frequency were determined， and cluster analysis of water absorption rate of TCM decoction piece was conducted according to closely related index as density，size，shape，moisture. TCM decoction piece with similar water absorption rate were bracketed together，so that of single ingredient TCM decoction piece can be estimated by water absorption of representative TCM decoction piece；the quantity of water evaporation and liquid extrusion were determined among different types of decoction machine（powered by electric and gas）；according to the above parameters，mathematical model of liquid volume of TCM medicinal broth prepared by different types of decoction machine had been established，and validated with TCM formula. RESULTS：Factors that affected the liquid volume added included the water absorption of each ingredient，the quantity of water evaporation and extrusion function. The mathematical model was liquid volume added＝water absorption of each ingredient×quality of decoction piece+the quantity of water evaporation+required amount of liquid－parameters of extrusion function×total weight of decoction piece；in validation test，the percentage of the practical amount of liquid to required amount was within ±5％. CONCLUSIONS：Established model can promote the accuracy liquid volume added and guarantee the quality of TCM decoction when using TCM decoction machine.