【1】work at强调从事某个工作或职业以及某个抽象目标或领域，表示在某个具体的工作岗位或领域上投入时间和精力

He's working at losing weight.他正在努力减肥。

He has worked at his craft for many years.他专研这门手艺已经很多年了。

Learning to play the piano isn't easy. You have to work at it .学钢琴不容易，你非得下功夫不可。

If you work at it, your English will improve.如果你努力下功夫，你的英语将会进步。

【2】work on强调进行中的工作或活动，表示致力于解决、处理、改进或完成某个具体的项目、任务或问题。

I’m working on a new report for the client.我正在为客户准备一份新的报告。

She is working on her speaking skills.她正在练习口语技巧。

Professor Bonnet has been working for many years on molecules of this type.  邦尼特教授多年来一直在从事这种分子的研究工作。

He is working on a new novel.他正在写一部新小说。

She's outside, working on the car.她在外面修理汽车。

You need to work on your pronunciation a bit more.你需要再加把劲改进发音。

用法区别总结如下

【1】“强调处理事情并解决问题”用“work on”

【2】“强调在事情持续花时间精力”用“work at”