As a necessary means of circulation of bulk cargo, ship transportation has made significant contributions to our country’s economic and social development. However, at the same, time marine diesel engines have brought severe problems of pollution emissions. In the context of increasingly stringent global emission limits, real-time monitoring of its emission parameters is of great significance to environmental protection, energy conservation, emission reduction, optimization of diesel engine control strategies and combustion performance. In recent years, tunable diode laser absorption spectroscopy (TDLAS) technology has gradually been favored due to its high accuracy and fast response. The wide application of this technology puts forward higher requirements for its research, so using mathematical software to simulate it has a certain value for this test system’s development and parameter adjustment. Combining the current ship emission detection problems, taking the most representative pollutant NO as the target gas, a simulation model of gas concentration measurement was made using software. The simulation model of the detection system is mainly composed of a light source modulation part, a line type function fitting part, a simulated gas absorption part, a line strength function S(T) fitting part, and a lock-in amplifier. The method of wavelength modulation is used to simulate the concentration measurement process. The high-frequency sine wave and the low-frequency saw tooth wave are superimposed to tune the laser. After the gas chamber absorbs the laser, the signal is adjusted by the lock-in amplifier to obtain the harmonic signals. The peak point of the value of the second harmonic divided by the first harmonic is used as the signal. The least-square method is used to fit the concentration-signal amplitude curve, and then the concentration inversion and error calculation are performed. The inversion error is within 2.5%. Analyzed the influence of temperature, pressure and other environmental factors on the signal amplitude and drew the harmonic graph. By introducing a reference gas chamber to eliminate environmental fluctuations on the results, there is no need to refit the concentration-signal curve when the environment changes. The result can be obtained directly. Tried to use different sine wave frequency, modulation coefficient and other parameters. Analyzed the influence of modulation parameters on signal amplitude and selected a suitable parameter range. It provided a certain reference for the construction of a diesel engine online emission test system and the selection of parameters.