Family functioning including family adaptability and family cohesion, and intraindividual reaction time variability (IIV) which serves as an index of attentional control has been found to relate to children's externalizing problems. However, it remains unknown whether family functioning interacts with children's IIV to predict their externalizing problems based on the diathesis-stress model. The present study examined this concern. Participants included 168 (M = 7.35 years, SD = 0.48; 48% boys) and 155 (M = 8.32 years, SD = 0.45; 49% boys) children at the first (T1) and second (after one year, T2) measurements, respectively. At T1, a flanker task was used to assess children's IIV. Mothers reported family functioning using the Chinese version of the Family Adaptability and Cohesion Scales, and children's externalizing problems using the Chinese version of the Child Behavior Checklist. At T2, mothers reported children's externalizing problems again. Results indicated that family functioning negatively and IIV positively correlated with children's externalizing problems. Furthermore, family functioning interacted with children's IIV to predict their externalizing problems concurrently and longitudinally. Specifically, low family functioning combined with greater IIV predicted prospective externalizing problems. Findings suggested that better attentional control (indexed by lower IIV) may buffer the negative effect of poor family functioning.