To measure CO2 gas in the exhaust of the D4114B diesel engine, the volume fraction and temperature of the gas were studied and analyzed. Based on the principles of Tunable Diode Laser Absorption Spectroscopy (TDLAS), the process of exhaust gas measurement was simulated using various modules in the SIMULINK library of MATLAB. Through simulation, the relative temperature error of the CO2 was 0.03%. The simulation results were verified with a D4114B marine diesel engine. A visualization window was added to its exhaust pipe and a corresponding test system was installed. A tunable laser with a semiconductor as a working medium was used as the laser light source to carry out on-line tests of the CO2 temperature in the gas emissions. In this study, the relative error of the test was less than 4.0%. It can be seen from the results that the different between the temperature measured by the model built with a SIMULINK and that of the actual diesel exhaust emission is slight. Therefore, the simulation result can be used as a reference for the diesel engine exhaust temperature measurement.