Suction caissons are widely used in the field of energy extraction, the internal and external wall penetration resistance is governed by the mechanical behaviour of the strength of suction caisson interface. Grain geometry was extracted from sand images by ImageJ, global outline coefficient, convexity and aspect ratio were adopted as shape descriptors. A series of Interface shear tests were conducted to investigate the effect of sand particle shape on the strength of the sand-suction caisson interface. The test results show that the increase in peak shear stress of the sand sample D1 (0.075~0.25 mm) with angular surface compared to the sand sample D5 (1.43~2.00 mm) with regular shape ranged from 16.98% to 72.23% under different normal stresses. The peak friction angle at the sand-steel interface increased with the increase of sand convexity; the peak friction angle at the interface decreased and then increased with the increase of aspect ratio. Smaller grain size sand tends to increase the peak friction angle of the interface, thus increasing the shear strength. The findings provide a basis for interface strength analysis of suction caissons.