This study presents an experimental investigation on the post-fire material properties of SN400B steel subjected to pre-strain. The specimens were pre-strained to the strains of 7% and 10%, and then heated to target temperatures of 300°C, 400°C, 500°C, and 600°C, where they were held for 30, 60, and 90 minutes. After heating, the specimens were water-cooled and subsequently subjected to a coupon tensile test at room temperature. The test result indicated that the strain ageing at temperatures between 300°C and 400°C led to a significant reduction in toughness. In contrast, heating at 500°C and 600°C resulted in a recovery of toughness due to the stress-relieving effect. These findings provide an experimental basis for assessing the reusability of steel structures subjected to post-earthquake fire scenarios, and underscore the importance of considering the combined effects of plastic deformation and fire exposure on structural design and post-fire assessment.