Abstract: There are lots of disputes on the pore genesis and its preservation mechanism in deep-ultra-deep reservoirs in petroliferous basins. Taking the ultra-deep Cretaceous Bashijiqike formation in the Kelasu tectonic belt of Kuqa depression in Tarim basin as an example, the paper makes an in-depth discussion. High-temperature high-pressure dissolution simulation experiments are performed to study the dissolution characteristics of the formation fluids under 3 different diagenetic environments. The results show that during the early diagenesis supergene period, the dissolution of CO2-rich meteoric freshwater was very significant, which was easy to dissolve feldspar or carbonate minerals and quartz was a little bit difficult to be dissolved. During the early diagenetic shallow-medium burial period, alkali dissolution in saline environment took the second place, and quartz, potassium feldspar and albite suffered from the dissolutions of different degrees. During the middle diagenesis period the dissolution of organic acid was weak, which was characterized by the dissolutions along fractures, gypsum dissolution and calcite precipitation. The dissolutions during the early diagenesis period are dominant. The surface porosities of reservoir residual intergranular pores and intragranular dissolved pores range from 2% to 4%, accounting for 50%~80% of the total reservoir porosity. For the reservoir with the burial depth over 5 500 m, the experimental simulation of the dissolution can reasonably explain the causes of the dissolved pores, which helps to fully understand the genesis of the ultra-deep high-quality reservoirs in the Kelasu tectonic belt and provides basis for the study on deep-ultra-deep reservoirs with multi-period of tectonic movements of the foreland thrust belts in the western basins of China