Line width is the key parameter in tunable diode laser absorption spectroscopy (TDLAS); the dominant line widths are Doppler and collisional broadenings. Under low absorbance, the ratios of 2nd to 4th harmonics at the line center monotonously decrease and intersect at a fixed point, regardless of the changes in Doppler and collisional line widths. Based on this characteristic, a method is proposed which employs the ratios to measure the line width. To validate the reliability and accuracy of the proposed method, the transition of CO2 at 6982.0678 cm-1 is selected to measure the line width, the gas partial and total pressure. Experimental results show that the proposed method can achieve high precision in actual measurements.