The fundamental electrochemical reaction mechanisms and the phase transformation pathways of layer-structured α-MoO3 nanobelt during the sodiation/desodiation process to date remain largely unknown. To observe the real-time sodiation/desodiaton behaviors of α-MoO3 during electrochemical cycling, we construct a MoO3 anode sodium-ion battery inside a transmission electron microscope (TEM). Utilizing in situ TEM and electron diffraction pattern (EDP) observation, α-MoO3 nanobelts are found to undergo a unique multi-step phase transformation. Upon the first sodiation, α-MoO3 nanobelts initially form amorphous Na x MoO3 phase and are subsequently sodiated into intermediate phase of crystalline NaMoO2, finally resulting in the crystallized Mo nanograins embedded within the Na2O matrix. During the first desodiation process, Mo nanograins are firstly re-oxidized into intermediate phase NaMoO2 that is further transformed into amorphous Na2MoO3, resulting in an irreversible phase transformation. Upon subsequent sodiation/desodiation cycles, however, a stable and reversible phase transformation between crystalline Mo and amorphous Na2MoO3 phases has been revealed. Our work provides an in-deepth understanding of the phase transformation pathways of α-MoO3 nanobelts upon electrochemical sodiation/desodiation processes, with the hope of assistance in designing sodium-ion batteries with enhanced performance.