In this study, in order to find out the mechanical properties of tight sandstones in Cretaceous Bashijiqike Formation of Kuqa Depression of Tarim Basin, combining with field problems during deep -ultra-deep oil and gas exploration and development, variations of mechanical properties of rocks were quantitatively studied using triaxial compression experiments under the effects of confining pressure and fluid and loading rate, and the reasons were also preliminarily analyzed. The results show that the maximum principal stress difference and elastic modulus of sandstones samples increase significantly with the increase of confining pressure. The microscopic reason is that the increase of confining pressure shortens the distance among particles inside the rock and enhances the cohesion of the rock, hence, particles are not easily separated. Sandstone samples experience the process of brittleness under low confining pressure→brittle-ductile transformation→ductile deformation under high confining pressure. Compared with dry sandstone samples, the reductions of elastic modulus for samples soaked in pure water, 150 g/L solution, 250 g/L solution, and 350 g/L solution were 67. 71%, 61. 45%, 64. 69%, and 57. 32%, respectively. The reduction for the sample soaked in pure water is the largest, and the increase of fluid salinity can weaken the trend of mechanical parameters of rock weakening. The crystallization and changes of electric double layer thickness are important controlling factors for the above changes. At a relatively low loading rate, the maximum principal stress difference, elastic modulus, and Poisson's ratio of the sandstone sample are small, but increase fast relatively with the increase of loading rate. When the loading rate reaches a certain critical value (e.g., about 0.05 mm/min in this experiment), the increase rate of mechanical parameters of rock slows down.