A bit allocation device includes: a channel estimation unit, performing a channel estimation to generate a plurality of channel qualities of a plurality of subcarriers; a processing unit, coupled to the channel estimation unit, generating a first plurality of bit numbers of the plurality of subcarriers according to the plurality of channel qualities; and a control unit, coupled to the processing unit, generating a second plurality of bit numbers of the plurality of subcarriers according to the first plurality of bit numbers and an upper limit of a total bit number.