In this paper, we study the effects of laser scanning step length and width characteristic on tunable diode laser absorption spectrum detection system, theoretically derive the principle of interaction between laser and gas absorption line, and analyse the principle of effects of step duration and height about scanning signal (ramp) on the laser central wavelength. After setting the simulation parameters, the curve between the number of ramp steps and the maximum scanning error is obtained. If the scanning signal has 4000 steps in one cycle, the error is less than 1‰ with full width at half maximum (FWHM) value being greater than 0.01 cm-1. The curves between laser linewidth and maximum amplitude or linewidth error are simulated, and also the relationship between laser linewidth and minimum FWHM is given with linewidth error maximum values being 1% and 0.5%. On condition that temperature coefficient n is 0.9 and air-broadened coefficient is 0.005, this paper gives the relationship among pressure, temperature and FWHM, from which the suitable pressure P and temperature T range are deduced. It can provide the relevant theoretical basis for selecting the laser and gas absorption lines and also for improving the system detection limit.