BiPO₄ with a phase junction was synthesized by calcinating hexagonal BiPO₄(HBIP) at different temperatures. The phase structures of BiPO₄ were gradually transformed from HBIP to monazite monoclinic BiPO₄ (nMBIP) and monoclinic BiPO₄ (mMBIP) with increasing calcination temperature. The enhancement of the photocatalytic performance and photocurrent were ascribed to the formation of phase junction. A nMBIP–mMBIP surface-phase junction was formed in BiPO₄ when HBIP was calcined at 500 °C, and its photocatalytic activity was 28.7 times as high as that of HBIP. Moreover, radical trapping experiments confirmed that the hole was the active species for BiPO₄ on the degradation of methylene blue.