Within　the　retrofitting　of　steel　structures,　welding　haunches　to　retrofit　weak　panel　zones　in　beam-to-column　joints　are　deemed　to　provide　reliable　cyclic　behavior　based　on　previous　tests.　Also,　the　force　transferring　mechanism　in　haunched　beam　sections　is　substantially　changed　under　the　influences　of　the　haunch　and　panel　zone.　For　the　investigation　of　the　behavior　of　the　haunched　beam　section　and　the　haunch,　finite　element　models　(FEMs)　were　established　with　considerations　of　material　and　geometry　nonlinearities　and　then　verified　against　previous　tests.　The　force　distributions　were　obtained　and　also　predicted　by　mechanical　models.　Results　showed　that　the　moment　resistance　and　initial　rotational　stiffness　increased　by　34.3%-76.9%　and　51.6%-83.3%　through　retrofitting.　Moreover,　a　noticeable　difference　in　force　distributions　was　observed　in　the　haunched　beam　section.　The　direction　of　the　shear　force　within　the　shear　plate　was　reversed　and　its　value　could　be　even　greater　than　the　beam　load.　After　the　yielding　of　panel　zones,　the　force　redistribution　appeared　and　subsequently　led　to　increased　forces　in　the　shear　plate　and　haunch　flange.　Furthermore,　the　comparisons　among　mechanical　models　indicated　that　the　AISC　model　might　underestimate　the　contributions　of　both　the　haunch　flange　and　the　shear　plate　in　the　elastic　stage,　and　the　plane-section-assumption　model　showed　better　agreements.　However,　without　considering　the　force　redistribution　in　the　post-yielding　stage,　the　above　two　models　underestimated　forces　by　8.8%-32.0%　and　9.7%-64.2%,　respectively,　and　the　simplified　model　was　proposed　with　better　predictions　concerning　the　impact　of　the　force　redistribution　to　provide　a　greater　safety　margin　in　design.