Nonlinear Raman spectrum of various of liquids is detected by using pulsed photoacoustic method. The cause of limiting the detect sensitivity of PARS techenique, mainly due to linear absorption, is discussed. Furthermore, an improvement method, differential method, is used to elimnate the disturbance from linear absorption. The detect sensitivity of PARS techenique is increased, so that the Raman spectrum of low concentration matter and the weakline of Raman spectrum can be detected. The probability of using differential PARS techenique to detect contaminant is discussed.