Cone-straight nozzle has been widely used in well bore cleaning, assisting drilling in petroleum industries due to its good clustering properties. The structure including cone angle and throat length has also been studied by scholars and been optimized. However, the internal flow properties have not been investigated clearly especially the boundary layer flow. In this paper LES model is used to capture the small-scale flow state near the nozzle wall. The RNG k-epsilon model is used to validate the accuracy of the LES simulation, the simulation data shows a good agreement. Three different inlet velocities are considered in simulations. The velocity distribution, shear stress, boundary layer thickness, skin friction coefficient and Reynolds stress are analyzed, the boundary layer separation and transition are discussed. The state of flow inside nozzle is laminar with inlet velocity of 1 m/s and gradually transferred into turbulent with the increasing inlet velocity. The most severe turbulence is at the entrance of the throat section, the vortex structure appears at the entrance of converging section and dose not survive, the vortex structure appears in sequence along the throat section wall when the inlet velocity is set to 5 m/s and 10 m/s the flow properties along the conical nozzle are revealed clearly, the main flow resistance is mainly produced in throat section. All these works aim to provide theoretical support for the further processing optimization of the nozzle structure and reduce the flow resistance of nozzle.