Purpose　-　The　purpose　of　this　study　is　to　develop　a　new　positioning　method　for　remotely　operated　vehicle　(ROV)　in　the　nuclear　power　plant.　The　ROV　of　the　nuclear　power　plant　is　developed　to　inspect　the　reactor　cavity　pools,　the　component　pools　and　spent-fuel　storage　pools.　To　enhance　the　operational　safety,　the　ability　of　localizing　the　ROV　is　indispensable.　Design/methodology/approach　-　Therefore,　the　positioning　method　is　proposed　based　on　the　MEMS　inertial　measurement　unit　and　mechanical　scanning　sonar　in　this　paper.　Firstly,　the　ROV　model　and　on　board　sensors　are　introduced　in　detail.　Then　the　sensor-based　Kalman　filter　is　deduced　for　attitude　estimation.　After　that,　the　positioning　method　is　proposed　that　divided　into　static　positioning　and　dynamic　positioning.　The　improved　iterative　closest　point-Kalman　filter　is　deduced　to　estimate　the　global　position　by　the　whole　circle　scanning　sonar　data　in　static,　and　the　relative　positioning　method　is　proposed　by　the　small　scale　scanning　sonar　data　in　dynamic.　Findings　-　The　performance　of　the　proposed　method　is　verified　by　comparing　with　the　visual　positioning　system.　Finally,　the　effectiveness　of　the　proposed　method　is　proved　by　the　experiment　in　the　reactor　simulation　pool　of　the　Daya　Bay　Nuclear　Power　Plant.　Originality/value　-　The　research　content　of　this　manuscript　is　aimed　at　the　specific　application　needs　of　nuclear　power　plants　and　has　high　theoretical　significance　and　application　value.