The presence of microstructures on a substrate has a great effect on the heterogeneous nucleation of water droplets. A circular conical apex and a cavity are adopted as the physical model to represent the typical defects which exist widely on substrates, and classic nucleation theory is used to quantitatively analyze the nucleation capability of different microstructures at different condensation conditions. The results indicate that conical cavities with narrower cone angles can reduce the nucleation free energy barrier as compared with apexes and a planar substrate, yielding a relatively higher nucleation capability. With the vapor pressure and supersaturation increasing, the nucleation rate increases rapidly, and some of the cavities that are originally not preferred for nucleation gradually translate into active nucleation sites. Consequently, the activated nucleation sites are finite for practical substrates under certain nucleation conditions, and the nucleation sites number density can be affected by the condensation conditions and the distribution of micro cavities on the substrate. The analysis also indicated that it is possible to realize spatial control of nucleation sites by the construction of micro cavities, and the nucleation sites number density can be intensified by increasing the amount of micro cavities on the substrate.