In order to enhance the function of reactive power compensation of a single-phase photovoltaic system in grid connection and realize precise detection and compensation of reactive and harmonic current of the single-phase grid-connected inverter， an improved method of detecting instantaneous reactive and harmonic current in the single-phase grid-connected photovoltaic system is presented. Based on instantaneous reactive power theory， algorithms of current decoupling in the single-phase system are derived for performing the reactive power control of single-phase photovoltaic grid-connected inverter. The method proposed effectively solves the difficulties of the current detection in single-phase connection. The paper proposes to adopt a deadbeat control strategy and derives the algorithm of deadbeat PWM. With this system used in the photovoltaic grid-connected inverter， not only the system has the function of supplying active power， but also can effectively perform reactive power compensation and harmonic suppression.