Photoacoustic spectroscopy (PAS) based on tunable diode laser (TDLPAS) is a widely recognised method for its excellent performance in trace gas monitoring and spectral analysis. This spectral technique provides many advantages such as high selectivity and sensitivity, on-line, real time and contactless measurements in a compact experimental arrangement. In the present paper, a resonant PA spectrometer based on a low-power near infrared distributed feedback (DFB) diode laser and a home-made resonant PA cell operating on its first longitudinal mode is developed, and the PA system has the advantage of simple experimental setup, easy operation, and low-cost, room temperature operating, etc. By investigating the performances of the PA cell at first, a good agreement was achieved between experimental and theoretical results; we have systematically investigated the influence of molecular relaxation effect on the PA detection of CO2 molecule near 1.573 microm, and presented the relevant analysis in theory. The research result shows that the detection sensitivity of the system based on PAS technique can be effectively improved by using the molecular relaxation effect.