Different crystallization mechanisms may co-exist during the crystallization of Cu₂ZnSn(S,Se)₄ (CZTSSe) film. To improve the properties of CZTSSe film, coordination between different crystallization mechanisms should be accomplished, which is challenging work. Herein, synergistical optimization of liquid-intermedia-phase assisted crystallization and NaSe_x assisted crystallization of CZTSSe film is achieved by simply adjusting the Se temperature of selenization. On one hand, the S-to-Se substitution of the film can be delayed by decreasing the Se temperature. This can facilitate the formation of liquid-intermedia-phase, thereby improving the bulk crystallization of the CZTSSe film. On the other hand, the decreased Se temperature can optimize the properties of NaSe_x flux by altering the reaction between Se and sodium source, thus improving the surficial crystallization of CZTSSe film. By regulating the above two crystallization mechanisms, the crystallinity, morphology and defect properties of CZTSSe film are improved simultaneously, which yields CZTSSe solar cells with the best efficiency of 12.3% (without anti-reflection coating). This paper reveals the complex mechanisms that how the Se source affects the crystallization of CZTSSe film, the results show the prospect of coordinating different crystallization mechanisms for high efficiency CZTSSe solar cell.