空气质量作为衡量一个城市环境质量的重要指标之一The air quality as one of the significant targets measuring a city’s environment quality OK?一直以来都是北京奥运会的焦点话题，The air quality as one of the significant targets measuring a city’s environment quality has always been the focal topic of Beijing Olympics.

不是一个完整的句子,建议改成
It is always a hot topic of the Beijing Olympic Games that the air quality should be set as one of the major indicators measuring a city’s environmental quality.

As one of the major indicator measuring a city's environment quality,the air quality has always been a focus topic for the Beijing Olympic Game.
The air quality,as one of the major indicator measuring a city's environment quality,has always been a focus topic for the Beijing Olympic Game.