The invention provides an instantaneous calculating method for detecting an output direct current component and an output harmonic component of an inverter. Firstly, a carrier cycle is taken as a unit to sample the output amplitude and the phase position of the inverter, and an exponential moving average filter (EMA) is used to implement filtering processing on sample values. Then the sample values are added one by one, the sum of alternate current components in the sample values is infinitely approximate to zero when the sampling times is infinite, so that the average value of the output direct current component of the inverter is obtained. Finally, the direct current component and an output reference value of the inverter in the sample value are subtracted to obtain the instantaneous value of the output harmonic component of the inverter. The output direct current component and the output harmonic component of the inverter, caused by various reasons, are instantaneously calculated, and the instantaneous calculating method is suitable for a single-phase inverter, a three-phase inverter, an off-grid inverter or a grid-connected inverter.