Abstract: Based on finite element method with concrete damaged plasticity model, a comprehensive study was carried out to investigate the effects of steel pipe diameter d, wall thickness of pipe t, diameter of pile D, length of pile H, and water cement ratio of grouting on the mechanical properties. The reliability of numerical simulation results was verified by test results. The results showed that with the increase of ratio of the diameter of steel pipe to pile d/D, the ultimate bearing capacity of the micro-steel-pipe piles increased correspondingly, and the critical values of d/D, which caused to the ultimate bearing capacity changed remarkably, were 0.28 and 0.72. Furthermore, the three dominant failure modes were indicated, they were steel pipe yield failure, steel pipe with outer grouting failure and grouting outside pipe failure. And then, the theoretical calculation formula for calculating the ultimate bearing capacity of micro-steel-pipe piles in three stages was obtained, and the theoretical calculation results were in good agreement with the experimental results and numerical simulation results. When the steel pipe diameter was the same and the thickness of the steel pipe wall increased, the axial load of the pile and the axial strain curve of the steel pipe tended to be softened for thinner wall of pipe, and then shifted to be ideal plasticity and hardening characteristics, and the ultimate bearing capacity of micro-steel-pipe pile increased linearly with the increase of the wall thickness. When the pipe diameter and the diameter ratio of the steel pipe to pile were the same, the grouting compressive strength was reduced by about 45%, the ultimate bearing capacity of the pile was only reduced by about 7.1%, it indicated that the grouting strength had no significant effect on the bearing capacity of the micro-steel-pipe piles.