The invention discloses a method for calculating a harmonic current in a dq rotating coordinate system. The method comprises the steps of: A, acquiring a grid voltage and a nonlinear load current, and transforming the grid voltage to obtain a corresponding voltage component and transforming the nonlinear load current to obtain a corresponding load current component in a stationary coordinate system; B, calculating instantaneous power of a nonlinear load by adopting an active power and reactive power calculation formula under isovector transformation; C, acquiring dq rotating coordinate system components of the nonlinear load current containing a harmonic component according to power conservation and isovector transformation; D, and filtering the current obtained in the step C to obtain a fundamental current component, and subtracting the fundamental current component from the dq rotating coordinate system components of the nonlinear load current separately to obtain harmonic current components in the dq rotating coordinate system. The method disclosed by the invention is used for calculating the components of the harmonic current in the dq rotating coordinate system, so as to provide a new control algorithm for APF.