Ionic liquids (ILs) have been applied in many fields in recent years, but the high viscosity of ILs has become an important factor hindering the further implementation of ILs. ILs/water binary mixtures often show a lower viscosity than pure ILs. However, the general effects of the water content on the viscosity remain unclear. In this work, the viscosities and self-diffusivity of a series of [C(n)mim][Ac] (1-alkyl-3-methyl imidazole acetate) ILs were computed by molecular dynamics simulations with a water mass percentage of 0.02-10%. The results show a general rule that the viscosities of all [C(n)mim][Ac] ILs (n = 1-6) first increase and then decrease by adding a certain amount of water, which correlate well with the corresponding reciprocals of self-diffusion coefficients. Interestingly, the inflection points of the viscosity gradually move forward in the direction of a lower water content when increasing the chain length, which is the result of increased hydrophobicity. Structural analysis shows that the chainlike structure of anion center dot center dot center dot water center dot center dot center dot anion is the key reason for the increase-to-decrease behavior of viscosity when water is added. Moreover, the sharp peaks of radial distribution functions between water and Ac- go up first and then move down, and the peak positions follow the same trend of inflection points.