Surface electric field of catalyst is widely recognized as one of the key points to boost catalytic activity. However, there is still a lack of convenient ways to tune the surface electric field to selectively boost the catalytic conversions of different types of reactants in specific catalytic reactions. Here, we introduce a conceptually new method to tune the surface electric field of electrode materials by adjusting the number and density of heterojunctions inside. Both theoretical and experimental results prove that the well-designed surface electric field of an electrocatalyst plays a key role in facilitating pre-adsorption and/or activation of reactants for selective conversion of trash ions to useful products in hydrogen and oxygen evolution reactions as well as NOx - reduction reactions.