burn down与burn up：

我个人认为二者的区别不大

(1)burn down: destroy (sth.) or be destroyed by fire 烧毁

The city was burnt down by the enemy.这座城市被敌人烧毁了。

A fire burnt the house down in half an hour.大火在半小时内将房子烧毁了。

(2) “烧毁”的另一个短语为：burn up＝destroy ( sth. )or be destroyed completely by fire

意为“(被)烧光”。

Eg  He burnt up all the old letters. 他把所有的旧信都烧光了。

The fire burnt up more than $50 000 worth of goods.大火烧掉了价值5万美元的货物。

(3) burn down 和 burn up 在以上例句中都作及物动词用，它们还都可以作不及物动词用。

Eg. Let's leave the fire to burn down and go into our tents.

让火自己慢慢熄掉，我们到帐蓬里去吧。

The spaceship burned up when it re- entered the earth's atmosphere.

宇宙飞船在重新进入地球大气层时烧毁了。