During Czochralski crystal growth, interface flipping is a frequent and concealed destructive phenomenon that induces interface instability and accumulate defects, ultimately degrading crystal quality. However, even for the widely-used Czochralski method, the interface flipping of a growing boule is unobservable. Therefore, presenting interface flipping process in the extreme high-temperature and sensitive crystal growth environment is crucial for understanding crystal growth, optimizing growth process, and hence improving crystal quality. Herein, benefitting from the time series analysis of growth interface electromotive force (GEMF), the interface flipping process during lithium niobate crystal growth was observed; and the quantitative relationship between GEMF trajectory and heat and mass transfer during interface flipping was revealed. Our GEMF method, applicable to the widely used melt growth furnace, provides real-time determination for interface flipping and offers feedback for interface control as well.