In this paper,the authors introduce a definition of the Schwarzian derivative of any locally univalent harmonic mapping defined on a simply connected domain in the complex plane.Using the new definition,the authors prove that any harmonic mapping f which maps the unit disk onto a convex domain has Schwarzian norm ‖Sf‖ ≤ 6.Furthermore,any locally univalent harmonic mapping f which maps the unit disk onto an arbitrary regular n-gon has Schwarzian norm ‖Sf‖ ≤ 8/3.