A resonant converter and burst mode starting method thereof are provided. The resonant converter includes a converting stage, a transformer to induce the output of converting stage from the primary side to the secondary side, a rectifying stage to rectify the output of the transformer, a filtering and load stage to filter the output of the rectifying stage, and a charge pump circuit coupled to the transformer or the rectifying stage. The charge pump circuit is for raising a voltage level of the filtering and load stage to higher than a predetermined value so that a feedback voltage of the resonant converter varies. The burst mode of the resonant converter is than started in accordance with the variance.