The propagation of paraxial light beams injected at an inclined angle in a planar waveguide with finite width is studied by numerical method. The soliton beams with different wavelength and same width，if their incident point and incident slanting angle are the same，will propagate along the same periodic Z-type route in the planar waveguide. With the power of the beam increasing，the distance between two adjacent reflecting points will become longer and longer with the propagation distance and the Z-type route will lose its periodicity. When the power of a beam become fairly strong，the beam will propagate along z direction of the waveguide，and will no longer be reflected back and forth between the two boundaries of the planar waveguide. Using the characteristic that a beam injected at an inclined angle in the waveguide will propagate along different route with different power，a power switch and an optical time division demultiplexer are designed.