过眼云烟
A flash in the pan.
Ephemeral grandeur.

eg：The public's enthusiasm to the plan turned out to be a flash in the pan.

中英俚语俗语来源：苏轼·宝绘堂记：“譬之烟云之过眼，百鸟之感耳。”

同义词：昙花一现 Sudden glory soon goes out