A channel estimation method is configured to estimate a channel length. The method includes the following operations: receiving an input signal; summing the input signal and an analog echo cancelation signal decrease an echo of the input signal, and generate a first signal according to a result of the summation; providing an analog gain value to the first signal to generate a second signal; performing an analog-to-digital conversion to the second signal to generate a third signal; obtaining a ratio according to an energy of a first frequency and an energy of a second frequency of the third signal; and estimating the channel length according to the ratio, and setting the analog gain value according to the estimated channel length.