The low core recovery is the main problem in single tube drilling. That tiny mineral particle of the core being taken away by drilling fluids continually causes the deficiency of core. The reasons for low core recovery is studied from the points of velocity profile, erosion action, core recovery and cuttings removal, and a set of single tube split-flow drilling tool has been designed with flow-distributing and area increasing to improve core recovery. The 3d-fem numerical simulation shows that this modified drilling tool can reduce the velocity of drilling fluids around core by about 25% under the condition of timely cuttings removal and solve the low core recovery.