A signal adjusting circuit is provided. The signal adjusting circuit includes a first operational unit, a second operational unit, an auto-gain controller (AGC), a first clamp circuit, and a second clamp circuit is provided. The first operational unit performs an operation to a digital signal and a first gain value, to obtain a first adjusting signal. The second operational unit performs an operation to the digital signal and a second gain value, to obtain a second adjusting signal. The AGC generates a third gain value according to the first adjusting signal. The first clamp circuit receives and restricts the third gain value between a first upper limit and a first lower limit for generating the first gain value. The second clamp circuit receives and restricts the third gain value between a second upper limit and a second lower limit for generating the second gain value.