A direct measurement system of OH reactivity for vehicle exhaust measurement (LP-LIF-kOH) was established based on the principle of laser-induced fluorescence in this work, then the rotary drum test on the exhaust of typical diesel vehicles was carried out and the total OH reactivity of heavy diesel vehicles at different emission levels was obtained. In the system, a pulsed UV laser is used to photolyze ozone to generate OH radical in a flow tube, and a pulse laser with a central wavelength of 308 nm is used to excite OH radicals. By using laser induced fluorescence technique, the decay of OH concentration caused by the reaction with the sampled reactive gases in the flow tube was measured and recorded, and the total reactivity of OH (kOH) with the sample gas can be derived by fitting the decay curve with an exponential function. In the closure experiment of the active organic compounds in the diesel vehicle exhaust gas, the measurement of the kOH of diesel cars in China emission standard III and China emission standard V was carried out. It is found that the value of diesel exhaust kOH decreases with the increase of China emission standard, and the kOH measured under cold start is greater than that measured under warm start for the same vehicle. The observed kOH is mainly contributed from NOx (55%), CO (2%), and the primary emission of VOCs (34%). A method of estimating the HC emission factor of diesel vehicles by using kOH is proposed to show the necessity of reactivity measurement in vehicle emission.