Abstract: Annular fuel element is a new type of fuel element proposed by Massachusetts Institute of Technology. The fuel element structure adopts double channel cooling structure with internal and external cooling simultaneously. Compared with the traditional rod-bundle fuel element, the annular fuel element with both inner and outer cooling channels has the advantages of high core power density and low fuel temperature, so it is of great significance to study its thermal-hydraulics characteristics. Computational fluid dynamics (CFD) method was used to analyze the flow boiling in the inner and outer cooling channels of annular fuel elements. A 2×2 annular fuel element model was established, and its flow and boiling heat transfer characteristics were studied based on Lee phase transition model and SST k-w turbulence model. The surface temperature of fuel rod, coolant temperature field in flow channel and gas volume fraction distribution were analyzed. The flow characteristics of the coolant were evaluated by calculating the secondary flow velocity distribution. The boiling heat transfer characteristics of the coolant were analyzed by calculating its heat transfer coefficient. The results show that the maximum secondary velocity region appears near the wall of the fuel rod, and the maximum secondary velocity increases with the increase of coolant temperature, but the average secondary velocity decreases with the increase of temperature. The temperature distribution of the annular fuel element outflow channel shows a trend of high temperature at the gap and low temperature at each sub-channel. Increasing the inlet temperature reduces the heat dissipated by the fuel rods through the inner passage and increases the heat dissipated through the outer passage. Increasing the heat flux will result in high temperature zones between adjacent fuel rods. At the same position in the axial direction, the surface temperature of solid fuel rod changes with a cycle of 90° along the circumferential direction, and the maximum temperature occurs at positions with circumferential angles of 45°, 135°, 225° and 315°. The average heat transfer coefficients at 0°, 90° and 180° positions are 22.53%, 52.16% and 7.9% higher than those at 270° positions, respectively. The heat transfer effect at 90° is the best. The overall heat transfer coefficient of the fuel element decreases sharply at the inlet due to the inlet effect. The coolant then boils in the channel to produce a large number of bubbles. Due to the randomness of bubble generation and movement, strong disturbance is formed to the surrounding flow, which makes the heat transfer effect increase and shows regular fluctuation.