I-mode is an attractive candidate among the confinement regimes proposed for burning operation with good energy confinement similar to H-mode but poor particle confinement similar to L-mode, resulting in no obvious impurity accumulation and free of large edge localized modes. Although I-mode has been recently achieved experimentally on several tokamaks, theory and simulation remain insufficient to explain the mechanisms behind the I-mode formation and its peculiar transport behaviour. In this work, an electrostatic gyrokinetic simulation using gyrokinetic toroidal code (GTC) is carried out for the typical I-mode plasma profiles. Linear simulations reveal that two competitive instabilities coexist within the range of short and long wavelengths during I-mode phase, respectively. It is found that the passing electron response cannot be treated adiabatically for those modes with mode numbers close to that of weekly coherent mode, which is considered to be a major player in the I-mode formation and responsible for the unusual transport behaviour exhibited in I-mode plasmas. Nonlinear simulations yield a turbulent heat transport level that is comparable to the experimental level, and about a half of the experimental particle transport level.