Abstract: The optical signals are converted into electrical signals of same time characteristics by the technique of the in-situ spectroscopic detection. The switch-on time is obtained by calculating the time difference of the exciting and electrical signals. Results show that the proposed method can effectively measure the switch-on time of DC and HF AC plasma antennas. The switch-on time of plasma antennas driven by DC and kHz AC power supplies are about 1 ms, and that of plasma antennas driven by MHz power supplies are approximately 0.5 ms. The error of 50 Hz AC plasma antenna is very large due to the unstable discharge state. The value of switch-on time is greatly related to the exciting power and the response time of the power supplies. In a certain range, with the increase of discharge power, the switch-on time of the antenna is shortened. Besides, the error is reduced by about 1%-2%.