The noise of photoconductive detector is so weak that the PAR 124A lock-amplifier is main test facility despite of discontinuation by long-gone manufacturer for decades. The paper uses 124A and 7124 lock-in amplifier system to test noise and response signal of several photoconductive detectors while use the SR830 internal oscillator and thermal noise of pure resistance as standard signal and noise source respectively. The results indicate that the data of two test system can fit each other except the background noise. The 124A lock-in amplifier with 116 transformer is 0.2nV/√Hz and 7124 lock-in amplifier with 5184 preamplifier is 0.8 nV/√Hz at 1kHz. The impedance of 116 transformer is small and the impedance of 5184 preamplifier is 5MΩ, so the signal of 116 transformer will decay and the 5184 preamplifier won’t in case of testing the performance of photoconductive detector with larger source resistance. Finally we suggest to use 7124 lock-in amplifier system in case of testing photoconductive detector with larger source resistance and use 124A lock-in amplifier system prior to 7124 lock-in amplifier system in case of testing photoconductive detector with small source resistance.