Water　hydraulic　piston　motor　is　one　of　the　important　transmission　components　in　a　water　hydraulic　system.　For　lack　of　enough　experimental　information　to　analyze　the　reliability　in　water　hydraulic　motor　design,　the　convex　model　is　chosen　as　wear　reliability　model　for　hydraulic　friction　pairs,　which　is　more　economical　and　reasonable　than　the　interval　model.　The　non-probabilistic　reliability　index　based　on　the　convex　model　is　established　using　first-order　Taylor　series　expansion,　and　the　non-probabilistic　reliability　model　of　wear　reliability　analysis　for　the　friction　pair　of　swash-plate/slipper　in　water　hydraulic　piston　motor　is　introduced　based　on　the　convex　model.　For　the　non-probabilistic　reliability　model,　only　the　bounds　on　uncertain　parameters　are　needed,　instead　of　probabilistic　density　distribution　or　statistical　quantities.　Numerical　examples　are　used　to　illustrate　the　validity　and　feasibility　of　the　presented　method.