吃力不讨好
A thankless task; do a hard but thankless job.

【发音】chī lì bù tǎo hǎo

【释义】费力而得不到好效果。
            Work hard but get little result.

【例1】改卷子是吃力不讨好的任务。
           Grading papers is a thankless task.

【例2】 要是你想帮助他这个人，那将是吃力不讨好。
             If you try to help him, you will be a fool for your pains.