be good at…指某人擅长于（做）某事,at在这里是一个介词,可以在其后跟名词,be good at sth.指擅长于某事；或在其后跟动词-ing形式,表示做某事做得很好,很出色,其同义短语是do well in.
要注意at和in的固定搭配.
She is good at English.她英语学得好.
He does well in Chinese.他语文学得好.