Phase-only　spatial　light　modulator　(SLM)　based　on　liquid　crystal　on　silicon　(LCOS)　is　a　kind　of　device　based　on　electrically　controlled　birefringence　effect　to　realize　phase　modulation.　Due　to　its　low　cost,　programmable,　high　resolution,　fast　response　time,　LCOS　has　been　widely　used　in　multi-channel　imaging,　adaptive　optics,　diffraction　optical　elements　(DOEs),　dynamic　holographic,　optical　tweezers　and　other　fields.　It　is　necessary　to　numerically　evaluate　the　modulation　characterization　of　LCOS　before　application.　Firstly,　the　phase　modulation　characterization　of　the　LCOS　(PLUTO　HED6010XXX　by　Holoeye　Company)　was　measured　based　on　the　Twyman-Green　interferometer,　and　the　curves　of　both　phase　shift　and　normalized　intensity　as　grey　level　functions　were　obtained.　Experimental　results　indicated　that　phase　modulation　of　the　LCOS　could　be　achieved　to　3.99　pi,　and　the　root-mean-square　value　(RMS)　of　normalized　intensity　was　less　than　0.01,　which　demonstrated　that　LCOS　could　be　regarded　as　a　phase-only　modulation　device.　This　method　is　also　suitable　for　the　evaluation　of　modulation　characterization　of　other　LCOS　devices.　Secondly,　a　phase　pattern　of　thin　lens　written　onto　LCOS　was　demonstrated.　Because　of　the　pixel　structure　of　LCOS,　the　theory　of　discretization　of　lenses　was　studied.　Both　simulation　and　experimental　results　were　obtained.　The　experimental　results　proved　that　the　convergence　character　of　the　lens　written　onto　LCOS　was　similar　to　optical　lenses.　In　the　experiment,　the　measured　focal　length　was　in　a　good　agreement　to　the　theoretical　deduction,　and　the　relative　error　(RE)　of　which　was　below　1%.　Both　simulation　and　experimental　results　showed　that　LCOS　could　be　used　as　lens　to　converge　the　plane　wave,　and　replace　the　optical　lens　successfully.