NOx is the most harmful component for ecological environment in diesel engine emission. As global environmental problem becomes increasingly serious, the NOx emission of diesel engine has been concerned widely. With a variety of emission regulations legalized, this not only presents new demands for NOx emissions of diesel engine but also puts forward higher requirements for test method. Feasible test methods usually are the methods of sampling and analysis, which need a long sampling time and can’t achieve online measure of emission. The online measure of diesel engine emission is not only to the benefit of supervising but also provides data support for optimizing combustion process and optimizing control strategy for diesel engine. Optical detection technique serves as a new detection method. As one of it, the tunable diode laser absorption spectroscopy has been widely applied in other areas because of simple experimental principle and fast response speed. In order to verify the feasibility of this method in diesel engine NOx emissions testing, experimental team researched on the online test of diesel NOx emission by TDLAS. Aiming at the main component of NOx, two interband cascade lasers whose tunable wavelengths were 5 262~5 265 and 6 136~6 142 nm were selected as detecting light source for respectively testing NO and NO2. Thorlabs PDA10JT type thermal detectors were chosen for collecting optical signal. The diesel engine of D4114B by the direct detection was researched and the test result with the result of AVL emission analyzer was compared. The measure error is less than 5％. In addition, experimental team conducted NOx transient emission measurement of a diesel variable conditions with the high time resolution of TDLAS and obtained a reasonable changed consequence of NOx emissions. Thus, appling TDLAS technique has important significance for transient emission characteristics of diesel engine.