Recent studies have confirmed that dense building clusters located in urban areas significantly changed ground motion. This phenomenon, known as site-city interaction (SCI) effects, have received increased attention. However, most of the current studies on the SCI effects were performed by assuming the linear behaviour of soil response, despite the significant influence of soil nonlinearity on the site and structural response. This article investigates the effects of the nonlinear behaviour of soils on SCI effects, through a series of numerical results. A new parameter named PSCI (percentage of SCI effects) was proposed to quantify SCI effects, which presented the percentage of ground motion modification caused by SCI effects. The parameter study revealed a significant influence of soil nonlinearity on SCI effects. The results suggested that soil nonlinearity affected SCI effects mainly through two opposing mechanisms: “damping increase” which reduces SCI effects; “modulus decrease” which increases SCI effects. A relationship between the nonlinearity-induced modification of SCI effects and the degree of nonlinearity was established.