  The inner drift chamber of the BESIII is encountering serious aging problem
after five year's running. For the first layer, the decrease in gas gain is
about 26% from 2009 to 2013. The upgrade of the inner tracking detector has
become an urgent problem for the BESIII experiment. An inner tracker using CMOS
pixel sensors is an important candidate because of its great advantages on
spatial resolution and radiation hardness. In order to carry out a Monte Carlo
study on the expected performance, a Geant4-based full simulation for the
silicon pixel detector has been implemented. The tracking method combining the
silicon pixel inner tracker and outer drift chamber has been studied and a
preliminary reconstruction software was developed. The Monte Carlo study shows
that the performances including momentum resolution, vertex resolution and the
tracking efficiency are significantly improved due to the good spatial
resolution and moderate material budget of the silicon pixel detector.
