Incompatible herb pair Gansui-Gancao is recorded in “eighteen incompatible” medicaments in many monographs of TCM (Traditional Chinese Medicine) which means the two herbs can not be co-used in most cases. However, Gansuibanxia decoction composed of Gansui(Kansui), Banxia(Pinellia), Shaoyao(Peony) and Gancao (Liquorice) is a traditional Chinese formula which has been clinically employed for the treatment of cancerous ascites, pleural effusion, peritoneal effusion, etc. The purpose of the study was to investigate the pharmacokinetics of main bioactive components in Gancao to explore the reasons why Gansui-Gancao can be used in Gansuibanxia decoction. A simple, rapid and sensitive UHPLC-MS/MS method for simultaneous determination of liquiritigenin, isoliquiritigenin, liquiritin, glycyrrhetinic acid and glycyrrhizic acid of liquorice in rat plasma was developed and validated. After extraction from plasma, the analytes and internal standard were separated on a C18 column with the mobile phase consisting of 0.1% acetic acid containing 0.2 mM ammonium acetate in water and acetonitrile via gradient elution. The electrospary ionization source was adopted under the multiple reaction monitoring mode. The method was succesfully applied to a comparative pharmacokinetic study of main bioactive components of Gancao in rat plasma after oral administration of the extracts of Gancao (GC), Gansui-Gancao (GS-GC), Shaoyao-Gancao (SY-GC), Gansui-Shaoyao-Gancao (GS-SY-GC) and Gansuibanxia decoction (GSBXD), respectively. The pharmacokinetic parameters had significant differences (P < 0.05) in different groups which showed that Gansui decreased the bioavailability of Gancao, while Shaoyao increased the bioavailability of Gancao. Hence, these may be the pharmacokinetic mechanism of incompatible herb pair Gansui-Gancao and the reasons why the herb pair can be used in Gansuibanxia decoction.