Abstract: Static random access memory (SRAM) may be subjected to the synergistic effect of total ionizing dose (TID) effect and single event effect (SEE) in space, resulting in the sensitivity change of single event upset (SEU) in the device. In this paper, the influencing mechanism of synergistic effect of TID and SEE on sensitivity of SEU in 90 nm SRAM was studied through integrated simulation at device level and circuit level by using technology computer aided design (TCAD) and simulation program with integrated circuit emphasis (SPICE). It was found that the SEU sensitivity increased with the increase of total dose when TID and SEE acted on the device at opposite working status, i.e., when the opposite data was stored. Whereas, the SEU sensitivity decreased with the increase of total dose when TID and SEE acted on the device at the same working status, i.e., when the same data was stored. The main reason was that after the damage of one of the pull-down NMOSes in SRAM by total dose radiation, the circuit recovery time and feedback time changed, and the recovery process and feedback process had different contributions to SEU sensitivity. This work may provide some reference for the radiation resistance design of memory devices.