Unburnt carbon detection in fly ash is the key target in fly ash using. Comparative studied three algorithms, FFT frequency domain analysis, autocorrelation, virtual digital lock-in amplification based on cross-correlation. The results show that the data of using virtual digital lock-in amplification measurement detect standard fly ash carbon content was more accurate and reliable, the deviation and variance were relatively smaller, less than 3%. Measuring the carbon content fly ash sample of 0.5%-4%, photoacoustic signal voltage value vs. carbon content was a more intuitive and clear linear relationship. Virtual digital lock-in amplifier technology can detecting the unburnt carbon content in fly ash accurately, simply and fastly . It’s great significant for energy conservation and environmental protection.