Abstract: To study the complex terrain of valley cities and its wind fields and pollutants diffusion, computational fluid dynamics (CFD) and complex terrain grid generation technique are used to establish the numerical simulation model, realizing application of CFD in complex terrain air movement and pollutants diffusion. To set up the boundary in CFD, the digital terrain of the study area was obtained by extracting altitudes using Google Earth elevation extraction software and then imported into Gambit software. In Gambit, the computational meshes over complex terrain were established using journal files written in program. In the CFD model, the turbulence was modelled by using the large eddy simulation (LES), and the pollutant dispersion patterns were described by the mixture multiphase model. The validity of the proposed approach was verified by the good agreement between the simulated distributions of air pollutant in Lanzhou City and monitoring results. The result showed that the complex terrain has a great impact on the wind field. For example, wind speed showed strip distribution characteristics due to the role of the blocking effect of mountains. The wind is weak at the back of the mountain. Meanwhile, the wind fields will determine the pollutants diffusion, as the divergence of the wind fields can affect the pollutant diffusion scope and pollution level. Under the given northwest wind conditions that air speed is 5 m/s above 10 m from the ground, the concentration of industrial air pollution on flat terrain is diluted about 10 times at the distance of 2.2 km from the pollution center. With mountain blocking, the pollution diffusion distance at which pollution concentration diluted 100 times is about one third of that distance in the relatively flat area, indicating that mountain blocking can inhibit the longitudinal diffusion of pollutants. In addition, the different inlet wind directions would change the interaction between the wind and the mountain, which will change the ground wind speed and local wind fields, resulting in the variations of the pollutant diffusion and distributions. Studies have shown that CFD method is feasible to study the influence of terrain on wind fields and pollutants diffusion.