【Objective】The relationship between sugar accumulation in grape berries and soil water potential was studied to provide a theoretical basis for water management at the maturity stage of grapes. 【Method】4-year-old ‘Fenghou’ grape （V. vinifera × V. labruscaa）， cultivated under root restriction method， was selected to consecutively determine soil water potential and daily sugar unloading rate of grape berries during veraison stage using berry-cup method. 【Result】Sugar unloading rate in grape berries was found highest at soil water potential ranging from -2 to -4 kPa （6：00-10：00）， and became relatively slower when soil water potential ranged from -5 to -14 kPa （11：00-18：00）. The sugar unloading rate increased initially and then declined when soil water potential reached to -15 to -19 kPa （19：00-4：00）， and was found lowest at about -20 kPa （5：00）. Gaussian average curve fitting showed that sugar unloading rate in grape berries was highest at -16.86 kPa followed by a decrease with decrease in water potential. 【Conclusion】-16.86 kPa was found as critical value of soil water potential for irrigation of grapevine at veraison stage.