This study investigated the microstructure transformation observed in an aging diesel -contaminated soil after thermochemical treatment (DT 150 degrees C + PS ) to explore its impact on engineering reusability. Three thermal remediation procedures (i.e., DT 150 degrees C , DT 350 degrees C , and DT 550 degrees C ) were selected as the control group. The results show that: (a) Pyrolytic carbon was produced in the DT 350 degrees C and DT 550 degrees C , while none was produced in DT 150 degrees C and DT 150 degrees C + PS ; (b) Iron -based minerals and organic matter in DT 150 degrees C + PS , DT 350 degrees C , and DT 550 degrees C were combusted and decomposed to release the Fe(II) substances; under stronger oxidation environments, Fe(II) substances would further transform into more stable Fe(III) substances; and (c) Halloysites and illites were formed in DT 350 degrees C , palygorskites and cordierites were formed in DT 550 degrees C , and oxidation in DT 150 degrees C + PS produced the sulfate minerals. The formed sulfate minerals in the DT 150 degrees C + PS sample filled pores and provided the skeleton strength, resulting in high unconfined compressive strength and poor permeability. Using a self -developed assessment model, only the DT 150 degrees C + PS sample showed an improvement (calculated as 2.96 x 10 - 5 ) in engineering performance and other methods led to the deterioration of soil mechanical properties. Thermochemical treatment is more suitable for engineering reuse, and this study can provide a theoretical basis for evaluating the greener reusability of contaminated soil after thermal or thermochemical remediation.