This study investigates the effects of self-circulating casing treatment with different radial inclination angles (tilted toward the rotor rotation direction) on the performance and stability of a high-load 1.5-stage transonic axial fan through multi-passage unsteady numerical simulations. Six inclination angles (0°, 20°, 40°, 60°, 80°, and 85°) were analyzed. The results indicate that as the radial inclination angle increases, the stall margin improvement capability of the casing treatment first strengthens and then weakens, while its negative impact on the fan's peak efficiency gradually diminishes. With the increase in the radial inclined angle, the bleeding mass flow rate of the self-circulating casing first increases and then decreases, and its capability to suction low-energy fluid in the rotor tip passage follows the same trend. However, airflow extraction rate alone does not solely determine the stall margin enhancement. Additionally, the adverse effects of the casing treatment on the stator passage below 80% blade height first intensify and then reduce as the inclination angle increases. Overall, the maximum stall margin improvement is achieved at an inclination angle of 80°, demonstrating an optimal balance between stability enhancement and efficiency preservation.