Compared with traditional solid fuel rod, annular fuel rod has double cooled surfaces. There is a phenomenon called heat split which the heat of annular fuel is conducted inward and outward. In this paper, the sub-channel analysis code is compared and verified by the hydraulic characteristics test and the critical heat flux test of the annular fuel assembly. Based on sub-channel analysis code, the heat split characteristics of annular fuel under different conditions were studied, such as different flow distribution ratio for internal and external, single-phase or two-phase heat transfer. The variation law of heat split in pre and post critical heat flux (CHF) conditions was obtained. In pre CHF condition the thermal resistance at the gap between fuel pellets and cladding is dominant, and the change of heat split for internal and external is small. In post CHF condition, the convective heat transfer heat resistance increases, and the heat split changes accordingly for CHF happens on different locations as internal or external.