NOx is the main pollutant substance of diesel engine. It is extremely harmful to human being and environment. NO content takes up more than 90 percent among NOx. Therefore, reasonable and accurate detection NO component of diesel engine emissions is particularly critical for the detection of NOx component. Current detection method can not reflect the real situation in time because of using sampling analysis. As a new testing technique, optical detection method has been applied in many fields. This paper combines the optical detection method to investigate TLAS technique for NO concentration under steady and transient process. By simulating the situation of NO concentration changes, the feasibility of this method to apply in the diesel engines verified. An inter-band cascade laser whose center wavelength is 5263 nm is chosen for detection. A gas compounding system is devised to obtain different concentration mixing gas. Matching the test sample gas of different concentrations with high purity NO as the measured gas is carried out with N2 as the background gas. In the steady measurement, respectively match the concentrations of 500, 1 000 and 2 000 ppm to conduct experiment. A relative error is less than 1%, and the maximum absolute error is 115 ppm. In the transient measurement, experiment is conducted respectively in the process with concentration increasing and decreasing. Besides the situation of the concentration change in the chamber is measured at real time. A maximum relative error of 52% is realized with a time resolution of 1ms. With the system we devised, a transient measurement of NO concentration can be realized. This method can be applied in the diesel engine, which is good for the study of the diesel engines emissions content.