Vertical　external　cavity　surface-emitting　lasers　(VECSELs)　have　unique　properties　such　as　excellent　beam　quality,　high　average　output　power,　and　high　repetition　rate.　Short　pulses　from　VECSELs,　especially　picosecond　and　femtosecond,　can　be　achieved　by　a　passive　mode-locking　mechanism　with　a　semiconductor　saturable　absorber　mirror　(SESAM).　We　theoretically　simulate　the　pulse　evolution　dynamics　in　VECSELs　and　investigate　the　influences　of　different　parameters　of　gain　and　SESAM　on　pulse　characteristics.　To　the　best　of　our　knowledge,　this　is　the　first　time　that　the　influences　of　fast　recovery　time　and　two-photon　absorption　coefficient　on　pulse　characteristics　have　been　studied.　©　2021　Optical　Society　of　America