This paper presents an integrated approach to analyse worst-case responding time (WCRT) of aperiodic message in WorldFIP for guaranteeing realtime requirement of distributed control system (DCS). Aperiodic message transmission within WorldFIP is a very complicated procedure because it is concerned with left time in a microcycle and must successively follow three different stages. To decrease pessimistic factors in WCRT of aperiodic message, we integrate the procedures of transmitting periodic message, identifying aperiodic request and transmitting aperiodic message together, and deduce a tightly bound for WCRT of aperiodic message. The result is validated by case study at last.