For understanding the influences of temperature and pressure on combustion diagnostics by the scanned-wavelength direct absorption spectroscopy(SDAS),the molecular absorption spectrum simulation method was used to discuss the application conditions.Based on HITRAN database,taking 7 185.597 cm^-1 water-vapor line as an example,the influence factors were analyzed at different temperature and pressure.Considering the modulation range of diode laser is about 2 cm^-1 at present,the application criterions of SDAS for combustion diagnostics were discussed and summarized,and the suggestions which broaden the application range were presented.