An orphogonal frequency division multiplexing(OFDM) based cognitive radio architecture was designed to meet the transmission requirements of a cognitive radio system. After finishing the allocation of subtransmission, and keeping BER within the limit that ensures service quality, an adaptive bit allocation method was proposed to allocate the bit and energy of subtransmission dynamically. The total power for launching needed reached its minimum (or SNR per bit reached its maximum). Compared with the fixed bit allocation method, the proposed method is simpler, decreases the system transmission power, and has better BER performance.