Abstract: Aiming at plot irrigation test which were designed and controlled by different lower limit of water content, conception of critical irrigation amount were proposed that could be used to judge whether irrigating was needed and calculate irrigation amount．In order to calculate irrigation amount conveniently, GUI was designed by Matlab language．Water demand law experiment of greenhouse beans was used to test soil moisture content data during pod setting period．Comparison between critical water quantity discrimination method and average method showed that irrigation discrimination and irrigation water quantity calculation results of two methods were significantly different, mainly due to difference in soil bulk density and field water holding capacity．This method was theoretically feasible and used Matlab programming to GUI design that was fast and convenient to calculate, with significant advantages．