A transmission power calibrating method and a system are provided for a wireless apparatus. The wireless apparatus are controlled by a computer host to be tested at several channels and gains, and a simplified transmission power calibration table is established accordingly. The simplified transmission power calibration table is compared with each of complete transmission power calibration tables in a database. A match is detected between a first complete transmission power calibration table and the simplified transmission power calibration table. When the match is detected, a mapping table is obtained from the first complete transmission power calibration table and stored in a memory of the wireless apparatus. When the match is not detected, the wireless apparatus is controlled by the computer host to establish a second complete transmission power calibration table from which the mapping table is obtained and the mapping table is stored in the memory of the wireless apparatus.