The intake process plays an important role in the operation of internal combustion engines. In the present study, a three-dimensional transient simulation of a four-valve diesel engine was performed using Large Eddy Simulation (LES) model based on software CONVERGE. The mean velocity components in three directions through the intake valve curtain, the flow separation around the intake valves, the influences of inlet jet on turbulence flow field and cycle-to-cycle variation were investigated in this work. The result shows that the mean velocity distributes non-uniformly near the valve curtain at high valve lifts. In contrast, the mean velocity distribution is uniform at low valve lifts. It is found that the flow separation occurs at valve stem, valve seat and valve sealing through the outlet of the helical port. In contrast, flow separation is only observed in the valve seat through the outlet of the tangential port. It is noted that the flow separation causes the backflow to occur near the valve, which affects the mass of fresh charge significantly. In order to research the influence of the inlet jet on turbulence flow fields, a parameter was proposed in this work, named Jet-Angle, which indicates the angle between the inlet jet centerline and the valve gap centerline. The results show that the orientation of the inlet jet changes regularly with crank angle and the turbulence flow field is influenced by inlet jet significantly. During the early stage of the intake process, the increase of the inlet jet velocity leads to large velocity gradient and strong turbulence intensity. In the latter stage of the intake stroke, the inlet jet velocity decreases and it results in the decrease of turbulence intensity. The cycle-to-cycle variation is weak during the initial stage of the intake stroke, and the cyclic differences intensify during the latter stage of the intake stroke, which is the result of the interaction of inlet jet and vortices in the cylinder.