The P(4) line of 3-0 band for CO gas in different pressures and concentrations was measured by employing tunable diode laser absorption spectroscopy combined with balanced difference detection technology. Due to the immunization to the common-mode noise of fluctuation of laser intensity, drifts of temperature and mechanical vibrations, the sensitivity of spectroscopy can be improved by balanced difference detection technology. In comparison with the direct absorption spectroscopy (DAS), the signal to noise ratio was improved by 3.4 times, and the minimum detection limit (MDL) reached 87 ppmv. The optimum pressure for CO was 70 Torr through comparing the spectral signals at 40 Torr, 55 Torr, 70 Torr and 85 Torr of 1% CO concentration. Moreover, different concentrations of CO at 70 Torr were detected by DAS and balanced difference detection, and the results demonstrated the balanced difference detection had a more linear relationship and the deviation was less than 5%. For further testification of the stability of the system, we collected the signals within 360 s for the Allan variance analysis, which denoted the most suitable detection time was 38 s and the MDL was 47.8 ppmv.