Passively　mode-locked　optically　pumped　vertical　external　cavity　surface-emitting　lasers　(　OP-VECSELs)　have　unique　properties,　such　as　excellent　beam　quality,　high-average　output　power　and　high　repetition　rate.　Many　applications,　including　frequency　comb　and　supercontinuum,　require　pulses　in　the　femtosecond　regime.　A　numerical　model　in　the　femtosecond　regime　is　essential　to　understand　the　pulse　formation　mechanism.　In　this　paper,　we　present　a　numerical　model　of　passively　mode-locked　VECSELs　in　the　femtosecond　regime.　We　analyze　the　influence　of　gain　parameters,　such　as　small-signal　gain,　saturation　energy,　and　the　gain　bandwidth　on　the　pulse　characteristics.　The　results　of　the　simulation　can　provide　a　direction　for　designing　the　gain　chip.