The width of a mixing zone between freshwater and seawater is important primarily because it directly reflects the extent of mixing and the growth and decay of the mixing zone indicates changes of the flow regime and water exchange between freshwater and coastal seawater. Wide mixing zones have been found in many coastal aquifers all over the world. However, the mechanisms responsible for wide mixing zones are not well understood. This work examines the hypothesis that kinetic mass transfer coupled with transient conditions, which create the movement of the mixing zone, may widen mixing zones in coastal aquifers. The hypothesis is tested by conducting two-dimensional numerical simulations based on a variable-density groundwater model for a scaled-tank model and a field-scale model. Periodic water levels, representing periodic tidal motion and freshwater table fluctuations, are imposed at the seaward and landward boundary, respectively, which cause the movement of the mixing zone. Both the scaled-tank model and the field model show that the combination of the moving mixing zone and kinetic mass transfer may significantly enhance the extent of mixing and create a wider mixing zone than the models without kinetic mass transfer. In addition, sensitivity analyses indicate that a larger capacity ratio (immobile porosity/mobile porosity) of mass transfer leads to a wider mixing zone, and the maximum width of the mixing zone may be reached for a given capacity ratio when the mean retention time scale in the immobile domain (the reciprocal of mass transfer rate) and the period of water level fluctuations are comparable.