In this paper, porous silicon (PS) was prepared by electrochemical etching method, its two-dimensional microstructure was observed by field emission scanning electron microscope (FESEM) and the three-dimensional topological image of PS was captured by Nano-Profilometry (NP), thus the reason for the difference of PS microstructures was discussed and the effect of microstructure on its mechanical property was investigated. Using MTS Nano Indenter XP, the relationship between hardness and Youngs modulus and the indentation displacement was studied and the mechanical property of PS with various porosities was compared. The experimental result showed that the porosity of PS prepared under various etching current densities (40,60,80 and 100mA/cm2) ranges from 60%—80%, which is increasing with the rising etching current density. The thickness of PS prepared under 20% HF is approximately 40—50μm; the average value of hardness and Youngs modulus of PS ranges from 0.478GPa—1.171GPa and 10.912GPa—17.15GPa， respectively； and the values decrease with the etching current density rising, and decrease or keep constant with the displacement increasing in the range of nano_hardness and micro_hardness, respectively. The impact on PS mechanical property of its surface condition, thickness, microstructure and environment was analyzed and the relationship between PS mechanical property and microstructure was obtained.