Organic-inorganic　hybrid　perovskites　have　attracted　extensive　attention　as　promising　photovoltiac　materials　for　high-efficiency　solar　cells.　In　this　study,　strain　effects　on　the　material　properties　of　Ge-based　perovskites　are　fully　investigated　by　the　first-principles　calculations.　The　results　indicate　that　the　structural,　mechanical,　electronic　and　optical　properties　of　CH3NH3GeX3　(X　=　Cl,　Br,　I)　are　sensitive　to　external　modulations.　The　band　gaps　of　three　Ge-based　halide　perovskites　are　well　predicted　by　using　the　HSE06　functional.　By　increasing　the　compressive　strain,　the　band　gaps　of　three　compounds　decrease.　A　suitable　band　gap　(1.36　eV)　of　CH3NH3GeI3　can　be　obtained　under　a　strain　of　-3%.　Moreover,　the　calculated　elastic　constants　further　imply　that　this　compound　is　stable　under　this　condition.　The　relationship　between　the　band　gap　variation　and　geometry　change　under　the　compressive　strain　is　revealed.　These　results　are　useful　for　understanding　the　effects　of　strain　on　the　material　properties　of　semiconductors　and　guiding　the　experiments　to　improve　photovoltaic　performance　of　Ge-based　perovskites.　(c)　2019　Elsevier　B.V.　All　rights　reserved.