It is well known that in magnetic confinement fusion research, the extraction current density of negative hydrogen ion source depends on the negative hydrogen ion density formed on the metal surface with low work function. Therefore, the formation of negative hydrogen ions is largely dependent on the dynamics of cesium in the source, and optimizing the performance of ion source to improve the extraction current density is a prerequisite. Based on the tunable semiconductor laser absorption spectroscopy, a new system for detecting cesium concentration in discharge chamber was designed and established to achieve rapid and real-time detection of trace Cs. The system design mainly focused on four aspects: light source generation unit, optical transmission testing unit, signal processing acquisition unit, and spectral display and analysis unit. And then an off-line testing system was established and system testing experiments were conducted. The curve of cesium concentration with temperature was obtained. As the temperature of the vapor cell increases, the detected Cs atom concentration exhibits a nonlinear positive correlation with temperature, demonstrating that the system enables quantitative detection of Cs atom concentration.