The system performance of tunable diode laser absorption spectroscopy (TDLAS) is affected by the characteristics of laser, the characteristics of detector, the modulation parameters, control circuit of laser and acquisition circuit of signal, etc. The characteristics of laser and detector are determined by the level of present production-manufacturing. For direct absorptive method, the modulation parameters contain scanning amplitude and scanning frequency. Scanning amplitude is adjusted to obtain complete absorption signal. The optimization and selection of scanning frequency are discussed in detail. Aiming at this problem, the influence of scanning frequency on direct absorption signals is observed by experiment based on a certain theory in the present paper, and the basis and method of scanning frequency optimization are summarized by analyzing the signal characteristics including amplitude, peak width, adjusted coefficient determination, signal-to-noise ratio and integrated absorbance, etc. The result of the experiment provides a definite basis for the selection of scanning frequency in TDLAS direct absorptive method.