The invention discloses a reactive power determination method and an apparatus for a power parallel capacitor set. The method includes: obtaining nameplate information of the power parallel capacitor set; obtaining the bus voltage of a connected bus of the power parallel capacitor set and reactive power measuring values of all relative branch circuits; calculating the reference capacity of the power parallel capacitor set according to the nameplate information and the bus voltage; in the conditions of effective reactive power measuring values, bus reactive power balance, and the deviation between the reference capacity and the nominal capacity in the credible range, the reactive power of the power parallel capacitor set is calculated according to the reactive power measuring values of the power parallel capacitor set, the reference capacity, and the nominal capacity; and in the condition of satisfying at least one of following conditions, the reference capacity is regarded as the reactive power of the power parallel capacitor set, and the conditions comprise ineffective reactive power measuring values, bus reactive imbalance, and the deviation not belonging to the credible range. According to the method and the apparatus, the reactive power of the power parallel capacitor set is calculated more accurately, and the requirements of various advanced applications can be met.