A novel method to stretch light pulses is proposed by using the free-space coupling technique, with which the ultrashort laser pulses can excite ultrahigh-order modes in the symmetrical metal-cladding optical waveguide with sub-millimeter scale. Because of high group-velocity dispersion of the ultra-high order modes' characteristic, the light pulses can be greatly stretched after propagation in the waveguide for a short distance. An analytical expression for Gaussian beam is found and it shows 1 ps light pulses can be stretched 1000 times after propagation in the waveguide for 1.26 mm.