Abstract: With gradually enhanced explorations in middle-shallow layers of petroliferous basin, deep layers become an important area for future petroleum exploration. In recent years, great breakthroughs have been made in deep exploration in the northwestern margin of Junggar Basin. With the increasing depth of exploration in the study area, the physical properties of reservoir gradually become poor, and the bottom boundary of hydrocarbon accumulation has attracted increasingly more attention. Based on geology, well logging, oil testing, analysis and assay data, a study on the bottom boundary of hydrocarbon accumulation in the northwestern margin of Junggar Basin is conducted using the method of irreducible water saturation, minimum flow pore throat radius and potential difference inside and outside reservoir, which is verified by the actual exploration results. The research results show that the bottom boundary of hydrocarbon accumulation in the northwestern margin of Junggar Basin has the porosity of 1.9 %-2.2 %, depth of 6 500-7 000 m, and flow pore throat radius of about 0.036 μm; below the bottom boundary of hydrocarbon accumulation, the free fluid space disappears in reservoir; the percolation capability of reservoir dies out; the potential difference inside and outside reservoir comes to zero, suggesting end of hydrocarbon accumulation, and no hydrocarbon exploration potential.