The steady propagation of an ultraintense laser pulse in a preformed narrow plasmas channel with the width in the order of laser wavelength is investigated, considering relativistic and ponderomotive nonlinearities and global charge conservation. A simplified two-dimensional model is derived to describe the structures of laser pulses and the channels and exact analytical solutions describing the cross-sectional structures of the laser amplitude and plasma density are presented. The influences of the channel width, plasma density, laser intensity and mode of electromagnetic field on laser pulse propogation in the plasma channel are also discussed. It is proved that if there is a preformed channel, the laser pulse can still propagate in the channel of pulse-duration order, even if the plasma density is much higher than the critical density (e.g. 20 times critical density). If the channel is wider, the plasma density is lower and when the laser intensity is higher, the laser pulses will propagate more easily. In the same channel condition, a higher laser intensity is needed for TE1 mode propogating at the same group velocity as for TE0 mode.