保持镇定

• keep a level head
• have a level head

例句

1. 并不是人人都能在危急时刻保持镇定自若的。
   It's not everybody who can keep as cool as a cucumber in an emergency.
2. 我们保持镇定、同心协力就没有问题。
   If we keep calm and stick together, we'll be all right.