Crystal phase is an intrinsic structural parameter to determine the physicochemical properties and functionalities of materials. The unconventional phases of materials with distinct atomic arrangements from their thermodynamically stable phases have attracted enormous attention. Phase engineering has recently made fruitful achievements in electrocatalysis field to optimize the performance of various electrochemical reactions. In this review, theoretical and experimental advances made in phase engineering of electrocatalysts are summarized. First, we introduce basic understanding on crystal phases of catalysts to show the dialectical relationship between bulk phase and surface catalytic layer, and highlight the multiple functions of phase engineering in catalysis studies. We then describe phase-controlled synthesis of materials through various experimental methods such as wet-chemical method, phase transition, and template growth. As a focus, we discuss the wide usage of phase engineering strategy in different kinds of electrocatalytic materials, and particular emphasis is given to establishment of reasonable crystal phase-activity relationship. Finally, we propose several future directions for developing more desirable electrocatalysts by rational crystal phase design.