Radio Frequency(RF) power amplifier is one of the key components that generate the Radio Frequency Fingerprint(RFF) of transmitter. The research on RF amplifier is a key breakthrough for RFF generation mechanism and individual identification. For this reason, an experimental method for RFF extraction of power amplifier is designed, which is called time domain RF Distinct Native Attribute(RF-DNA). The method successfully extracts the RF fingerprint of the power amplifier and the time domain RF-DNA fingerprint is visualized. The research results show that the RF fingerprint of the power amplifier is mainly reflected in the amplitude distortion characteristics. The time domain RF-DNA fingerprint generated by the instantaneous amplitude can achieve the correct classification of the amplifier individually. When the SNR is higher than 12 dB, the classification correct rate is above 91%. The visualized RF-DNA fingerprint can be intuitively observed, as well as the similarity and difference of statistical features among different power amplifiers.