This paper provides the comprehensive characterization of SEU sensitivities based on 3D-TCAD, SPICE simulations and the irradiation results. The impact of strike location on transient features are evaluated in simulation, and the influence of charge sharing effects on SEU thresholds of SRAM circuits is also analyzed. Additionally, the charge sharing effects are analyzed and verified by circuit-level simulation and irradiation experiment in 256 Kbit pulse-mitigated SRAM. The split charge injection simulation shows that the SEU threshold reduce about ~97% in the worst condition. The results provide the meaningful guidance for radiation hardening design of FDSOI integrated circuits.