First I tried the player_weaponstrip and doesn't work,knowing that it's not available in Portal.Then I use the trigger_weapon_strip instead,covering the block with "trigger texture" over the spawn (info_player_start) and still doesn't work.I even tried to activate it with a trigger_once.So what's the exact way to strip players' portalgun?

Make sure that you flagged "kill weapons" or something like that. Otherwise, the player will pick it up again.