A compensation circuit for compensating an input voltage offset of an error amplifier has a level shifter, a first trimming circuit, a second trimming circuit, and a compensation current sinking device. The level shifter shifts levels of a feedback voltage and a predetermined reference voltage and outputs a level shifted feedback voltage and a level shifted reference voltage. The first trimming circuit adjusts the level shifted reference voltage by trimming a first resistance thereof according to a trimming code, wherein the trimming code has the ratio relation of the input voltage offset and a resistance to be trimmed. The second trimming circuit adjusts the level shifted feedback voltage by trimming a second resistance thereof according to a trimming code. The compensation current sinking device sinks currents passing through the first and second trimming circuits.