The invention provides an optimal reactive power compensation method for three-phase imbalance at the low-voltage side of a 10kV distribution transformer, which comprises the steps that firstly sequence decomposition is carried out on three-phase current of a power grid through calculating the imbalance degree of the three-phase current so as to work out the positive sequence current, negative sequence current and zero sequence current in the power grid; the negative sequence current is required to tend to zero as far as possible in order to realize the optimal three-phase balance and distribution transformer loss indexes at the low-voltage side of a distribution transformer area so as to solve the optimal reactive power compensation capacity of a three-phase capacitor. The method realizesthe operation with the optimal three-phase balance and distribution transformer loss indexes at the low-voltage side of the distribution transformer area under the mode of pure capacitance compensation. According to the method, the optimal control strategy is calculated based on the real-time operation state of the distribution transformer at the low voltage side, it does not need to rely on thecontinuous switching of capacitor banks to slowly obtain the optimal indexes, the three-phase balance and distribution transformer loss indexes at the low voltage side of the distribution transformerarea are greatly improved on the basis of not increasing the cost of the existing low-voltage shunt capacitor compensation device, and the economic and social benefits are great.