The content of pollution gas in atmosphere is generally small, so the photoacoustic signal detected by photoacoustic spectroscopy to monitor is very weak. In this paper, on the basis of Holmes Duffing equation, we propose a new method i.e. variable scale difference method which is quite suitable for photoacoustic signal detection. This method can be used to detect weak signal by transforming the size of signal and then substracting the resulting signal. Theoretical analysis and experimental measurements show that it can well restrain the common-mode noise of system phase space, and it can also highlight the chaos state of critical value. This method has a relative error less than 5%, indicating that it can be effectively used to detect the amplitude of weak photoacoustic signal that has a higher frequency and unknown phase and frequency.