With the rapid development of 5G NR (New Radio), the explosive increment of traffic amount is calling the utilization of unlicensed band. 3GPP has proposed LAA (Licensed Assisted Access) to use LTE in unlicensed band and pointed out that NR-U (NR-Unlicensed) can reuse most designs of it. However, the existing channel access mechanism of LAA is conservative under the coexistence scenario of NR-U, which leads to the waste of time resource. To address the problem this paper proposes a hybrid channel access mechanism to take advantage of the LBT (Listen-Before-Talk) mechanism of LAA when channel is quite busy and transmit di-rectly with reduced power when it is relatively idle. The channel busy degree is judged by a series of periodically updated adaptive thresh-olds. System-level simulation verifies that under the coexistence scenario of NR-U the proposed mechanism can achieve higher UPT (User Perceived Throughput) and lower delay than other channel access mechanisms.