Nondestructive　estimation　of　residual　stresses　has　been　a　major　challenge　in　engineering.　Herein,　the　collinear　Lamb　wave　mixing　method　is　proposed　for　residual　stress　measurement　in　metal　plates　under　immersion　ultrasonic　testing.　Numerical　simulations　have　been　conducted　to　investigate　the　influences　of　excitation　parameters　on　the　performance　of　the　wave　mixing　method.　The　results　showed　that　the　receiver　position　and　cycle　numbers　of　primary　waves　affect　the　amplitude　of　the　method,　and　optimal　excitation　parameters　recommended.　Ultrasonic　immersion　experiments　are　conducted　on　two　types　of　plate-like　specimens　using　the　collinear　Lamb　wave　mixing　method.　Experimental　results　showed　that　the　proposed　method　is　effective　for　measuring　the　residual　stress　in　metal　plates.　The　immersion　ultrasonic　testing　can　significantly　reduce　the　influence　of　inconsistent　coupling　conditions　between　the　transmitters　and　the　specimen　on　experimental　results,　therefore　it　is　more　reliable　than　conventional　contact　testing.