A sampling method with adjusting duty ratios is provided and includes the following steps. A first working pulse signal which has a pulse-width duty ratio D in a switching period Ts is provided. A first adjusting period comprising first N successive switching periods of the first working pulse signal is set, wherein N is a natural number larger than 1. A second working pulse signal which has second N successive switching periods with their corresponding pulse-width duty ratio D1, D2, . . . , DN to drive the switch in the converter circuit is provided and the measured signal is generated, wherein the sum of D1, D2, . . . , DN substantially equals to N·D and the second N successive switching periods constitute a second adjusting period.