  $\alpha$ decay is a common and important process for natural radioactivity of
heavy and superheavy nuclei. The $\alpha$ decay half-lives for even-even nuclei
from Z=62 to Z=118 are systematically researched based on the two-potential
approach with a quasi-stationary state approximation. To describe the
deviations between experimental half-lives and calculated results due to the
nuclear shell structure, a hindrance factor related with $\alpha$ particle
preformation probability is introduced. Our results can well reproduce the
experimental data equally to the density-dependent cluster model and the
generalized liquid drop model. We also study the isospin effect of nuclear
potential in this work. Considering the isospin effect the calculated results
improved about 7.3$\%$.
