The choice of the parameters of the secondary sound sources including the number, the layout and the strengths of the sound sources are key parts in the design of active noise control system of power transformer. Considering the practical application environment of transformer noise control, an acoustic radiation model in half-free space is established. After selecting the sum of the squared sound pressures of the observation points as the objective function of the control system, the layout of secondary sound source is optimized by genetic algorithm and the optimal strengths of the sound sources are resolved by the analytical method. It is found from the optimization results that the distances of the horizontal and longitudinal space between two adjacent secondary sources will gradually decrease when the number of secondary sources increases, and the secondary sound sources array should be close to the primary sound sources as much as possible regardless of the number of secondary sound sources. Under the same condition of the secondary sound sources, the effect of noise reduction varies with the noise plane to be reduced. A 100 Hz single frequency experiment verifies the effectiveness of the optimized layout and the strength of the sound source.