To meet the requirements on well integrity of oil wells under large temperature difference environment，it is needed to clarify the evolution of mechanical properties of oil well setting cement under a wider range of temperature and pressure. Pure cement system and silica-cement system was selected as research targets，the influence of temperature，pressure and silica fume content on the evolution of cement strength under the temperature range of 30—200 ℃ was systematically studied and the driving mechanism of cement strength development and high temperature strength retrogression mechanism under each factor was investigated. The results showed that within 30—110 ℃，the compressive strength of pure cement system increased with the increase of temperature. As The temperature increased from 100 ℃ to 150 ℃，the strength retrogression of the pure cement system happened under high temperature. Within the temperature of 110—200 ℃，the ultrasonic intensity of silica-cement system changed not monotonically with the temperature；however，the ultrasonic intensity development could be accelerated by the increase of pressure. At the temperature of 200 ℃，the phenomena of strength retrogression and volume expansion of silica-cement system happened. By increasing the amount of silica sand，the early strength decline rate could alleviated to a certain extent. Phase composition analysis showed that the development of the early strength of cement was mainly related to the formation of C-S-H gel and Ca（OH）₂；at the temperature of 130 ℃，the main reason causing the high temperature strength retrogression of pure cement system was the formation of α-C₂SH；at the temperature of 200 ℃，the main reason causing the strength retrogression of silica-cement system was that the C-S-H gel was gradual transited from amorphous to other crystalline.