The contact, and thus the hole collection between Cu2ZnSnSe4 (CZTSe) and Mo, is a crucial issue to improve the performance of CZTSe solar cells. In this work, a method to improve the back contact is explored by spraying Na3PO4 on the surface of the Mo back contact. With the O provided from Na3PO4, extra MoO2 and MoO3 are formed at the surface of the back contact, and partial MoO2 is transformed into MoSe2 under high Se2 partial pressure during the selenization process. The formation of MoSe2 progresses from dispersed spots to a continuous layer but not from the reaction between CZTSe and Mo. Although a thick MoSe2 layer is formed, the CZTSe device performance increases from 7.2% to 8.3% on average. This study affords new insight into the formation of MoSe2, thus deeply strengthening the understanding of the back contact of kesterite solar cells and of two-dimensional chalcogenide devices.