This paper proposes a pseudo-potential describing slit flow in the formalism of lattice Monte Carlo simulation with the bond fluctuation algorithm as the unique basic micro-relaxation mode. The main characteristics of slit flow, such as the parabolic velocity profile and the pressure-flux relationship, are successfully reproduced in a three-dimensional self-avoiding multichain system. Both Newtonian and non-Newtonian regimes are revealed. The chain conformation and nonlinear rheological behavior are investigated. The simulation results agree with experimental measurements. This method call be used to investigate the viscoelastic properties at different layers as well as the global properties. Some peculiar phenomena in inhomogeneous flow are found to be consistent with previous theoretical predictions by others.