I want to delete all completed tasks in one time instead of delete completed tasks one by one.
I don't want to keep completed tasks with me.
Unfortunately, that's not possible right now, but we will consider adding such option in the future. You're welcome to vote for this request in our Votebox: https://todoist.com/Vote/showProposal/409/
Please note that your completed tasks are actually stored on the "Get completed tasks" list at the bottom of any project and can be deleted by clicking on the "Delete completed" link, but the "# Completed" section is just a completion history and will not affect the performance of our service regardless of how many completion events will be noted there.