Kesterite Cu₂ZnSn(S,Se)₄ (CZTSSe) shows great promise in thin filmphotovoltaic application due to its reasonable band gap, low cost and non-toxic earth-abundant composition. Post-selenization is the generally adopted approach to obtain CZTSSe film with large-grain size. Thoroughly understanding and well controlling the grain growth process during selenization is thus essential for achieving high-performance solar cell devices. Here we demonstrate that the absorber microstructure can be adjusted by altering selenization conditions, especially the heating rate and annealing time. It is found that heating rate determines formation manner of top CZTSSe layer, while annealing time affects both the crystallizationdegree of bottom CZTSSe layer and the selenization of Mo contact layer. Properly adjusting the annealing time at an optimized heating rate is proven to successfully eliminate the fine-grain layer and obtain high quality CZTSSe thin films with large-size grains. Based on the observations, growth mechanisms for the microstructure development during selenization of solution processed Cu₂ZnSnS₄ films are suggested. Finally, CZTSSe device efficiency of 9.24% is achieved at an optimal selenization condition.