In the late Northern Zhou (beǐ zhōu 北周,557-581AD) era,after the emperor Xuan Di (xuān dì 宣帝) died from disease,his prince took the throne later as the king of the country was very very young,hence needed someone to sponsor him. An ambitious and capable minister named Yang Jan (yáng jiān 杨坚) was qualified for this critical position,and before long,he became the actual regent of the hole government. Yang Jan had a sophisticated wife bearing a good family back ground. One day,his wife said to him:"Since Northern Zhou is dying. Now it is as if you are riding on the back of a tiger. It will be dangerous to dismount. You can do nothing but continue." Yang Jian thought this suggestion quite reasonable. Later,he founded the Sui Dynasty,and united China.

This idom is used as a metaphor meaning that one is in a stubborn situation and have no choice but to go on with it.

故事发生在北周晚期。在宣帝病死以后，他年幼的王子登机继承了皇位，大臣杨坚乘机辅佐小皇帝，并很快掌握了国家大权。杨坚的妻子劝告杨坚说：“北周已经不行了。你现在就像骑在老虎背上，形势逼迫着你不能下来，只好干下去吧！”杨坚觉得妻子的话很有道理，后来就干脆自己当了皇帝，建立了隋朝，统一了中国。

“骑虎难下”这个成语用来比喻做事中途遇到很大困难，但形势所迫，又不能中止。

骑虎难下 (qí hǔ nán xià)
【翻译】have a tiger by the tail.
【释义】骑在老虎背上不能下来。比喻做一件事情进行下去有困难，但情况又不允许中途停止，陷
            于进退两难的境地。
【例句】小张已经答应别人把这个项目做好，但现在出现了很多技术上的问题，真是骑虎难下啊！
【近义词】进退两难、欲罢不能
【反义词】一帆风顺、势如破竹