To realize online inflow mass flux validation during ground testing of a direct-connect scramjet facility, an absorption-spectroscopy-based system was developed and tested. First, a mass flux calculation method based on temperature and flow velocity measurements in the isolator section was presented. Taking into account environmental adaptability in vehicle testing and the system's long-term stability, miniaturized system and optomechanical design strategies were presented. Then, key algorithms, such as relative wavelength calibration, flow temperature, and velocity retrieval were described. Two independent vehicle tests with inflow states of Ma6.5 and 6.0 kg/s were carried out, and the results show that the maximum deviation in the tunable diode laser absorption spectroscopy results is 5%, validating the feasibility of the method. This research provides a new solution and tool for the delicate measurement of flow state with the potential to feed back the control inflow capture process.