The invention discloses a tester of an AC dual-power backup power automatic switching system based on high-voltage constant-voltage source power amplifier output. The tester comprises a signal processing given unit, and a push-pull output stage, a power amplification stage, a sample protector, an output network, AC output and a module power supply which are connected with the signal processing given unit. The tester is characterized in that the signal processing given unit is configured to conduct subtraction on AC small voltage signals of external input and AC voltage feedback signals of output of the power amplification stage to output amplified error signals. The push-pull output stage is configured to amplify positive half waves and negative half waves of the error signals. The power amplification stage is configured to conduct power amplification on the positive half waves and the negative half waves to output AC sine-wave voltages. The output network is configured to conducting capacitive and sensible phase compensation on loads of the AC sine-wave voltages. The sampling protector is configured to be disconnected from external given small signals and output voltages when theloads exceed a set voltage or current threshold.