In order to better build the neutral beam injector with negative ion source (NNBI), the pre-research on key technologies has been carried out for the comprehensive research facility for fusion technology (CRFFT). Cesium seeding into negative-ion sources is a prerequisite to obtain the required negative hydrogen ion. The performance of ion source largely depends on the cesium conditions in the source. It is very necessary to quantitatively measure the amount of cesium in the source during the plasma on and off periods (vacuum stage). This article uses the absorption peak of cesium atoms near 852.1 nm to build a cesium atom concentration detection system based on tunable diode laser absorption spectroscopy (TDLAS) technology. The test experiment based on the cesium cell is carried out, and obtained the variation curve of cesium concentration at different temperatures. The experimental results indicate that the system detection range is within 5×106—2.5×107 pieces/cm3 and the system resolution better than 1×106 pieces/cm3.