Abstract: The screw cast-in-place pile is a kind of variable-section cast-in-place pile with cylindrical top and threaded bottom. Due to its special construction technology and section form, the bearing mechanism of screw piles is different from that of piles with the same section. Through the indoor model tests, the failure modes of the screw piles and the straight rod piles are obtained intuitively by using the image processing technology, and then the bearing mechanism and the calculation of the bearing capacity are discussed, and the field tests are used for verifiction. The model and field tests show that the bearing capacity of a single screw pile is higher than that of the pile with the same diameter, and the lateral resistance of the screw pile is obviously higher than that of the pile with the same diameter. The mechanism of limit lateral resistance is different between the screw segment and the straight segment. For the screw section, under the limit state, because of the teeth of mechanical interlocking between the teeth and the teeth of the soils as a whole and the shear displacement of surrounding soils, the lateral resistance is shear strength of surrounding soils, the fracture surface is continuously arch-shaped, and the average diameter of the arch section is higher than the diameter of the screw teeth. For the straight pile, under the limit state, the lateral resistance is the friction resistance between the pile body and the soils around the pile, and the failure surface is the cylinder around the pile. According to the test results, the expression for the ultimate bearing capacity of a single screw cast-in-place pile is given, and the value of the relevant parameters are suggested.