Single Event Transients (SET) of linear device have the characteristics of transience and propagation, which cause great difficulties to fault location and mitigation design for on-orbit electronic systems and devices, and are significant threat to the reliability of spacecraft. To mitigate SET for linear devices, comprehensive experimental assessment on the SET is required, and parameters of the SET in the worst case especially need to be determined. Then specific filtering circuit can be designed according to the worst case parameters of SET and the application circuits. In this paper, SET assessment and mitigation on operational amplifier LM124 which is typically used in spacecraft are investigated. Firstly, SET characteristics of the device are evaluated by pulse laser facility, and the worst SET with duration of 35μs and amplitude of 7.2V are observed. Then, by using Hspice simulation, the SET of LM124 are compared and verified with experimental assessment, and mitigation on the worst SET using different parameters of filter circuits are investigated. Finally, the mitigation on worst SET with the optimized filtering circuit is investigated and verified again using the pulsed laser facility. The work of this paper is useful to investigate and mitigate SET of other linear devices applied in space.