杯水车薪（杯水車薪）　

◎ 杯水车薪 bēishuǐ-chēxīn

[a cup of water can‘t put out the fire on a carload of wood;try to put out a burning cartload of faggots with a cup of water—an utterly inadequate measure] 用一杯水去扑灭一车燃烧的柴草。比喻力量太小,无济于事

犹以一杯水,救一车薪之火也。——《孟子·告子上》