Tin-based　perovskites　are　promising　for　realizing　lead-free　perovskite　solar　cells;　however,　there　remains　a　significant　challenge　to　achieving　high-performance　tin-based　perovskite　solar　cells.　In　particular,　the　device　fill　factor　was　much　lower　than　that　of　other　photovoltaic　cells.　Therefore,　understanding　how　the　fill　factor　was　influenced　by　device　physical　mechanisms　is　meaningful.　In　this　study,　we　reported　a　method　to　improve　the　device　fill　factor　using　a　thin　cesium　iodide　layer　modification　in　tin-based　perovskite　cells.　With　the　thin　passivation　layer,　a　high-quality　perovskite　film　with　larger　crystals　and　lower　charge　carrier　densities　was　obtained.　As　a　result,　the　series　resistance　of　devices　was　decreased;　the　shunt　resistance　of　devices　was　increased;　and　the　non-radiative　recombination　of　devices　was　suppressed.　Consequently,　the　fill　factor,　and　the　device　efficiency　and　stability　were　greatly　enhanced.　The　champion　tin-based　perovskite　cells　showed　a　fill　factor　of　63%,　an　efficiency　of　6.1%　and　excellent　stability.　Our　study　reveals　that,　with　a　moderate　thin　layer　modification　strategy,　the　long-term　stability　of　tin-based　PSCs　can　be　developed.