Abstract: This work studied the thickening progression mechanism of the silica fume – oil well cement composite system at high temperatures (110–180 °C) in order to provide a theoretical guidance for the rational application of silica fume in the cementing engineering. Results showed that silica fume seldom affected the thickening progression of oil well cement slurry at 110–120 °C, but when temperature reached above 130 °C, it would aggravate the bulging degree of thickening curves and significantly extend the thickening time, meanwhile causing the abnormal “temperature-based thickening time reversal” and “dosage-based thickening time reversal” phenomena in the range of 130–160 °C and 170–180 °C respectively. At 130–160 °C, the thickening time of oil well cement slurry was mainly associated with the generation rate of calcium hydroxide (CH) crystal. The introduced silica fume would be attracted to the cement minerals’ surface that were hydrating to produce CH and agglomerate together to form an “adsorptive barrier” to hinder further hydration of the inner cement minerals. This “adsorptive barrier” effect strengthened with the rising temperature which extended the thickening time and caused the occurrence of the “temperature-based thickening time reversal” phenomenon. At 170–180 °C, the pozzolanic activity of silica fume significantly enhanced and considerable amount of C−S−H was generated, thus the “temperature-based thickening time reversal” vanished and the “dosage-based thickening time reversal” was presented.