In this paper, based on the complex random telegraph noise (RTN) data in FinFETs, the impacts of the trap coupling effect on RTN amplitude are studied statistically. The coupling effect is found to be enhanced by the double-side coupling mechanism in FinFETs. The non-monotonic V_g dependence of amplitude coupling strength is observed for the first time, which is contributed by the evolution of channel percolation paths. In addition, the impacts of HCI and NBTI stresses on the coupling strength are studied. Based on the new understandings on coupling effect in FinFETs, the impacts of amplitude coupling on typical digital circuits are also investigated. The results are helpful for comprehensive understanding of the RTN coupling mechanism in nanoscale FinFET technology and RTN-aware circuit design.