Abstract: An experiment was conducted to study the critical heat flux in the cavity injection system. A constant-width slice heater block was used for the representing of the reactor vessel lower head, in combination with a power-shaping approach, in the experiment. The critical heat flux（CHF） at various angular position of the external wall was measured under the pool-boiling condition and the forced cooling condition, respectively. The experimental results indicated that, under both pool-boiling condition and forced cooling condition, the variation in CHF at different angular position can be divided into three kinds. In the inlet region, the critical heat flux increases with the increasing of the angular position under pool-boiling condition, but under forced cooling condition the critical heat flux decreases with the increasing of the angular position. In the middle region, the critical heat flux increases with the increasing of the angular position. In the outlet region, the critical heat flux decreases with the increasing of the angular position.