Abstract: In the present work,a numerical simulation was performed to study the flow characteristics of lean jet to cross-flow in a main tube in the safety injection of reactor cooling system.The influence scope and mixing characteristics of the confined lean jet in cross-flow were studied.It can be concluded that three basic flow regimes are marked,namely the attached lean jet,lift-off lean jet and impinging lean jet.The velocity ratio V_R is the key factor in the flow state.The depth and region of jet to main flow are enhanced with the increase of the velocity ratio.The jet flow penetrates through the main flow with the increase of the velocity ratio.At higher velocity ratio,the jet flow strikes the main flow bottom and circumfluence happens in upriver of main flow.The vortex flow characteristics dominate the flow near region of jet to cross-flow and the mixture of jet to cross-flow.At different velocity ratio V_R,the vortex grows from the same displacement,but the vortex type and the vortex is different.At higher velocity ratio,the vortex develops fleetly,wears off sharp and dies out sharp.The study is very important to the heat transfer experiments of cross-flow jet and thermal stress analysis in the designs of nuclear engineering.