To have a deep understanding on the critical heat flux (CHF) characteristics of ex-vessel core catcher system, the present study conducted subcooled flow boiling experiments on an inclined downward-heating surface in a diverging channel. The effects of inlet mass flux (100 kg/m2s to 400 kg/m2s), subcooling (1 K to 30 K) and inclination angle (5° to 45°) on the CHF were investigated. A high-speed camera and pressure transducer were used to simultaneously record the two-phase flow behavior and pressure. The results show that with the increase of heat flux, the small bubbles grow and coalesce into a bubble blanket gradually. The condensation of bubble blanket outside the heating surface induces the reversal of downward liquid. Beyond the subcooling threshold (ΔTin ≥ 20 K for θ = 5° and ΔTin ≥ 25 K for θ ≥ 15°), a two-phase flow instability characterized by the pressure shock was observed, which leads to a remarkable enhancement in CHF. An increase in the inclination angle also leads to an increase in CHF, but with a declining slope. The effects of key parameters on the characteristics of the bubble blanket were further analyzed. Especially, the reduced inclined angle makes it easier for bubble blanket to form, and it's easier for the reversible and irreversible dry patches to be formed on the wall. The dry patch fraction is much larger so that the hovering time is much longer and the frequency is lower, so the CHF decreases. And then, a dimensionless correlation between the bubble blanket parameters and the thermal–hydraulic parameters (mass flux, subcooling and inclination angle) was developed.