Abstract: Compared to lanthanides, actinides have more accessible oxidation states in compounds and higher tendency to form covalent bonding interaction with ligands. Early actinides(actinium to americium) display rich redox chemistry as all or most their valence electrons can readily participate in forming chemical bonds. In contrast, the chemistry of late actinides(curium to lawrencium) is dominant by +3 oxidation state and behaves similarly to lanthanides. Accessing unusual oxidation states and developing redox chemistry of actinides have been sought by synthetic chemists. In this review, we will first analyze the electron configuration of actinides and define “unusual oxidation states of actinides”. Based on that, we will introduce the history and the recent accomplishments in expanding the unusual oxidation states of actinides. In this first part of review, we will focus on the unusual oxidation state of actinides in gas phase, solid state and aqueous solution. In addition to provide a survey of literatures, we will discuss the remaining challenges of this field and its great impact on fundamental bonding theory of actinides and the nuclear science.