The advantages of the LCL type filter are presented by the increase of the capacitance of the grid-connected converter. LCL filter needs smaller inductance value comparing to L type filter at the same performance in harmonic suppression. With the input and output harmonic current attenuation ratio as the optimization target, the designed parameters may larger than the actual demand. It would lead to the increase of size and cost. This paper analyzes pulse width voltage harmonics at the side of three-level grid-connected converter based on double Fourier series, At last a quantitative design method of LCL Filter of three-level PWM grid-connected converter is proposed, which achieves specific quantization of the filter performance. This method is not designed to reduce the harmonic content of the grid current, but to meet the Total Harmonic Distortion (THD) requirements as much as possible to reduce the LCL filter specifications and cost. Finally, simulation and experimental results verify the effectiveness of the design.