Based on tunable diode laser absorption spectroscopy, combined with wavelength modulation spectroscopy technology, different volume concentration CO2 was selected to be measured. The influence of modulation voltage and modulation frequency of sine wave on second harmonic signals was observed. Results show that the signals perform well when modulation voltage is 0.25 V and modulation frequency is 10 kHz. CO2 with different concentration was measured by using the single optical system set up in the laboratory. The results show that the correlation coefficient of the concentration and the peak value of the second harmonic signals is 0.998. Moreover the detection limit is 450 ppm. The experiment result provides definite basis for choosing modulation parameters of such system and can build foundation for industry application.