Recently, system-on-chips (SoCs) are increasingly employed in reliable applications for their high-performance and high-densities. Moreover, the structure shrinking of SoC leads to its proneness to radiation-induced soft errors. This paper presents a fine-grained fault injection framework for SoC (FFI4SoC) to assess the reliability of SoC against soft errors. FFI4SoC facilitates fault injection for SoC by defining the primary components and rules that are required by fine-grained fault injection. Furthermore, based on FFI4SoC, we develop a fine-grained fault injection tool named SSIFFI for bare-metal MicroZed. The design of SSIFFI is presented in order to illustrate the application of FFI4SoC. Finally, SSIFFI is engaged in simulated fault injection experiments to explore the cause of single event functional interrupts (SEFIs) and to validate functional properties of FFI4SoC. The experimental results disclose detailed reasons for SEFI and prove that FFI4SoC can be employed to assess reliability of SoC well with the merit of fine-grained injection.