1. An adaptive equalizer, comprising: a filter, configured to filter input signals according to a first filter coefficient to obtain a first filtered signal, and further configured to filter the input signals according to a second filter coefficient to obtain a second filtered signal; a coefficient updating unit, configured to buffer the input signals to obtain buffer signals, and further configured to obtain an update magnitude of the filter coefficient according to a level (n−1) error signal and the buffer signals, and obtain the second filter coefficient according to the update magnitude; a standard decider, configured to decide the first filtered signal based on an original constellation map to obtain a first decision signal, and further configured to decide the second filtered signal based on the original constellation map to obtain a second decision signal; a level decider, configured to decide the first filtered signal based on a level (n−1) constellation map to obtain a level (n−1) pseudo decision signal, and further configured to switch the level (n−1) constellation map to a level n constellation map if average energy of the level (n−1) error signal is less than a level (n−1) threshold, wherein the level n constellation map is the original constellation map; and a comparator, configured to compare the level (n−1) pseudo decision signal with the first filtered signal to obtain the level (n−1) error signal; wherein n is a positive integer greater than 1.