The pure kesterite CZTSe absorbers are vital for CZTSe solar cells. However, the secondary phases such as Zn-Se, Cu-Se, Sn-Se and Cu-Sn-Se, which are almost harmful to CZTSe solar cells, often appear in the CZTSe absorbers. To investigate the phase transition pathways of as-sputtered Cu-Zn-Sn-Se precursors during selenization can provide a basis for the reduction of secondary phases and the preparation of high-performance CZTSe solar cells. In the work, the sputtered Cu-Zn-Sn-Se precursors were annealed in a selenium-containing atmosphere at temperatures from 200 to 580 degrees C for 30 min. The phase transition pathways were analyzed and presented. Low or high selenization temperatures were unfavorable for CZTSe growth. When the temperature were below 530 degrees C, the crystallization would be insufficient. Meanwhile, when the selenization temperature were above 560 degrees C, the CZTSe films would decompose during selenization and degrade rapidly. 530 degrees C seemed to be a relatively appropriate selenization temperature although the grain size was still not large enough at this temperature. By continuously inletting H2Se gas during the whole selenization process, CZTSe decomposition could be suppressed even at high temperature of 550 degrees C. And thus the grain size was improved and CZTSe solar cell with the efficiency of 6.78% was obtained.