An algorithm of harmonic observer of various harmonic contents is proposed for real-time estimation in cycle nonsinusoidal signal, which is applied to neutral point balance strategy. This paper introduces the neutral-point balance strategy based on dynamic control parameters after analyzing the switch model of single-phase full-bridge Conergy NPC and space vector pulse width modulation (SVPWM) modulation methods. A harmonic observer for the capacitance and voltage of filtering, based on state observer theory, is designed in order to get information about harmonic signal and orthogonal signal from sampling signal. Meanwhile, the observed information will also be applied to the neutral point balance strategy. Both simulation experiments and hardware experiments verify the feasibility and validity of this balance strategy based on neutral-point potential in harmonic observer.