<p>To address the challenge of producing large-area ultrafine-grained titanium alloy sheets via conventional single-pass friction stir processing (FSP), this study employed multi-pass friction stir processing (MFSP) technology to fabricate large-area titanium alloy sheets and investigated the effects of low heat-input and high heat-input parameters on the microstructure, tensile properties, and superplasticity of Ti–6Al–4V alloy sheets. Under low heat-input parameters, both the processed zone (PZ) and interfacial zone (IZ) of the ultrafine-grained sheets exhibited similar microstructures composed of fine equiaxed grains, with average grain sizes of 0.47 and 0.45 <i>μ</i>m, respectively. The proportion of high-angle grain boundaries exceeded 93 pct in both zones, and the strength–ductility product reached 11.34 GPa· pct. Furthermore, the ultrafine-grained sheets demonstrated excellent low-temperature superplasticity within the 550 °C to 700 °C range, achieving a maximum superplastic elongation of 819 pct at 700 °C and a strain rate of 1 × 10⁻³ s⁻¹. Grain boundary sliding was identified as the primary deformation mechanism. In contrast, under high heat-input parameters, both the PZ and IZ consisted of lamellar structures. During tensile deformation, the synergistic effect of dislocation walls and nanotwins within the lamellar <i>α</i> phase resulted in an excellent combination of room-temperature tensile strength of 1354 MPa and elongation of 16.8 pct, with a strength–ductility product of 22.75 GPa· pct. Superplastic tensile tests revealed that the lamellar microstructure is less conducive to grain boundary sliding, resulting in significantly inferior superplastic properties compared to the ultrafine-grained sheets prepared with low heat-input parameters.</p>