How to support real-time traffic of important aperiodic messages using Foundation Fieldbus (FF) is investigated in this paper. An integrated worst-case response time (WCRT) analysis of aperiodic message indicates FF can't effectively guarantee real-time traffic of important aperiodic message despite FF providing such mechanisms as PT (Pass Token) priority and PT rotation period, etc. After finding the cause of current mechanism' deficiency, an improved mechanism proposed and validated with simulation in this paper. Further, some conclusions such as ongoing work are provided in this paper.