Two-dimensional particle-in-cell simulations are taken to study the interaction of a relativistic, circularly polarized laser pulse with a preformed overdense plasma channel containing a slice of micron size. The laser pulse is confined in the channel, so it can keep higher intensity on a longer time scale inside the channel than the case without a channel. The electrons, both in the slice and from the channel, are pushed forward in the channel by the large light pressure of the laser pulse, followed by the ions accelerated by the electro static field generated by the charge separation. As a result, the acceleration of the slice is more efficient and has a better collimation than in the case without a preformed channel.