The photoacoustic (PA) spectrum of methane in the concentration range of 5%—100% has been experimentally investigated by coustant direct current drive coupling mechanical chopper technology. The results indicated that abnormal signal saturation occurred at high gas concentrations. The main reasons and its correlated factors of the photoacoustic signal has been quantitatively analyzed based on signal absorption and photoacoustic principles. It is found that the change in coupling coefficient of sound source and optical cavity modes caused by the intensity of incident light absorbed by gas sample is the major reason, and a criterion was put forward for signal saturation depth estimation.