here is what I found - four steps -- make sense to me. The last step actually resolve your "外面水管高于地下室,怎么放干净" issue.
Protect outside pipes and faucets. In some homes, the outside faucet has a separate shut-off in the basement. If you have a separate valve for outside faucets, shut if off. Then go outside and turn on all the faucets to drain water in the lines. Leave the outside faucets on while you go back and check your outside shut-off valve for a small brass plug or cap on the valve. Turn this plug far enough that water drains from the valve. Then, tighten the plug back and turn off all the outside faucets.