This　paper　introduces　a　longitudinal　study　monitoring　occupants＇　window　opening　behaviour　in　a　mixed-mode　office　building　in　Beijing,　China,　when　natural　ventilation　is　specifically　used　for　controlling　the　building＇s　indoor　thermal　environment.　Based　on　the　field　measured　data,　the　influence　of　factors,　including　outdoor　air　temperature,　outdoor　PM2.5,　indoor　air　temperature,　time　of　day,　occupancy　and　previous　window　state,　on　the　observed　state　of　windows　is　analysed.　All　of　them　are　influential　on　occupants＇　window　opening　behaviour　in　the　case　study　building,　and　so　they　can　be　used　to　model　occupants＇　window　opening　behaviour　in　buildings　in　China　to　achieve　a　better　consideration　of　occupant　behaviour　in　dynamic　building　performance　simulation.