<p>The mechanical integrity of cement sheaths in oil and gas well production has garnered considerable attention due to global incidents of oil and gas leaks. However, limited instances have explored the correlation between bonding strength and the microstructure of the interfacial transition zone (ITZ) at casing-cement and formation-cement interfaces under high temperatures. This study employs a back-scattered electron scanning electron microscope and digital image processing to quantify the porosity of the bulk paste and the ITZ thickness at the cement-casing and cement-formation interfaces. To reveal the underlying mechanism of temperature and silica flour dosage on the compressive strength of hardened well cement paste and the bonding strength at the cement-casing and cement-formation interfaces, the relationship between the mechanical properties and the microstructure results of bulk paste, casing-cement interface, and formation-cement interface is analyzed across various silica flour dosages and temperatures. The results indicate that the addition of silica flour leads to an initial increase, followed by a decline, and finally an increase in the ITZ thickness at the cement-casing and cement-formation interfaces at 150&#xa0;°C. When the silica flour dosage is fixed at 0 wt.% or 50 wt.%, the ITZ thickness between the casing/formation and cement pastes increases with rising temperature. Furthermore, the bonding strength of the cement-casing and cement-formation interfaces decreases as the ITZ thickness increases.</p>