Generally, Gwakhyangjeonggi-san has been used for treatment of diarrhea-predominant irritable bowel syndrome. In this study, an ultra-performance liquid chromatography-electrospray ionization-mass spectrometer method was established for the simultaneous quantification of marker compounds 1-20 in Gwakhyangjeonggi-san water extract. All analytes were separated by gradient elution using two mobile phases on a UPLC BEH $C_{18}$ ($100{ imes}2.1mm$, $1.7{mu}m$) column and maintained at $45^{circ}C$. The injection volume was $2.0{mu}L$ and the flow rate was 0.3 mL/min with detection at mass spectrometer. Regression equations of the compounds 1-20 were acquired with $r^2$ values ${geq}0.9950$. The values of limit of detection and quantification of all analytes were 0.01-2.79 ng/mL and 0.03-8.37 ng/mL, respectively. The amounts of the compounds 1-20 in Gwakhyangjeonggi-san water extract were not detected $-3,236.67{mu}g/g$. The established LC-MS/MS methods will be valuable to improve quality control of traditional herbal formula, Gwakhyangjeonggi-san.