An online gas analyzer with taking HF gas detection as an example based on tunable diode laser absorption spectroscopy (TDLAS) technique is presented. In order to eliminate or at least reduce the measurement uncertainty and gain high reliability, a close-circle digital-control module (D-CM) with functions of digital signal generator, digital lock-in-amplifier, data acquisition and data preprocessing have been developed to substitute the multi-independent analog boards in our previous system. Meanwhile, the signal correlation method is used to lock the absorption wavelength to reduce the effects of the ambient temperature and other factors. Utilizing the digital-control module and the digital signal processing techniques in the field tests, the sensitivity and stability of the TDLAS system have been greatly improved. Furthermore, smaller volume, lower cost and weaker signal distortion are achieved.