In　essence,　safety　evaluation　of　damaged　mortise-tenon　joints　(MJs)　involves　checking　whether　the　joints　have　sufficient　residual　bearing　capacity.　However,　the　load,　which　is　necessary　in　the　calculation　process,　is　difficult　to　observe　in　practice,　resulting　in　insufficient　reliability　of　the　evaluation　results.　Thus,　this　study　proposes　the　concept　of　a　margin　of　bearing　capacity　(MBC)　of　an　MJ　and　presents　a　calculation　method　for　assessing　the　safety　of　one-way　straight　mortise-tenon　joints　(OSMJs).　In　the　calculating　process,　the　load　was　replaced　with　the　observed　apparent　damage　and　rotation　angle.　First,　the　MBC　includes　the　influence　of　joint　damage　and　rotation　angle　on　the　residual　moment　bearing　capacity　of　the　joint.　Second,　the　apparent　damage　to　an　OSMJ　is　simplified　to　average　section　loss　rates　in　the　height　and　width　directions,　which　reduces　the　amount　and　dif-ficulty　of　data　acquisition.　Third,　the　damage　model　of　an　OSMJ　included　in　the　proposed　calculation　method　establishes　the　transforming　relationship　between　the　measured　average　section　loss　rate　and　the　damage　setting　on　the　model,　and　avoids　the　influence　of　uneven　distribution　of　the　actual　size　reduction　on　the　calculation　of　the　residual　moment　bearing　capacity.　The　results　of　unidirectional　pushover　tests　of　several　damaged　and　intact　OSMJ　models　showed　that　most　absolute　errors　between　the　calculated　and　measured　MBCs　were　less　than　0.04,　indicating　the　accuracy　of　the　proposed　method　for　practical　safety　assessment　of　damaged　OSMJs.