This　paper　presents　a　robust　cascaded　control　strategy　to　underwater　robot　thrust.　The　dynamics　of　surge　motion,　of　propeller　axial　flow,　of　propeller　shaft,　and　of　electrically　driven　circuit　in　the　motor　constitute　a　cascaded　system　with　respect　to　propeller　thrust.　Instead　of　the　usual　parameter　perturbation,　generalized　modeling　errors　are　considered　in　the　plant,　which　may　be　parametric　errors,　ignored　high-order　modes,　or　some　unmodeled　dynamics　in　the　underwater　thrust　system.　External　disturbances　are　also　taken　into　account,　which　may　be　the　random　noises　from　mechanical　or　electrical　equipment,　or　the　environmental　forces　possibly　induced　by　nonuniform　currents,　ocean　internal　wave,　or　cable　tension.　Combined　with　state　feedback　control,　an　online　neural　network　(NN)　compensator　is　introduced　to　identify　the　modeling　errors,　while　L2-gain　design　is　used　to　suppress　the　externally　continuous　or　instantaneous　disturbances.　The　Lyapunov＇s　second　method　is　applied　to　instruct　the　controller　design,　which　guarantees　the　uniformly　ultimately　bounded　(UUB)　stability　of　the　error　system.　By　analyzing　the　tracking　errors,　it　is　recommended　how　to　properly　select　the　controller　parameters.　Good　tracking　performance　and　reasonable　control　inputs　are　illustrated　by　numerical　simulations.　©　1976-2012　IEEE.