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12-mm-thick Al–Li–Cu alloy plates were fabricated by stationary shoulder friction stir welding (SSFSW) to evaluate the softening behavior of the whole joint under a given rotational speed of 400 rpm and a welding speed of 60 mm/min. Continuous dynamic recrystallization, discontinuous dynamic recrystallization, and geometric dynamic recrystallization are the main dynamic recrystallization mechanisms in SSFSWed joints. Such mechanisms help to gradually change rolling grains of base material (BM) into equiaxed grains of welding nugget zone (WNZ) and significantly lower dislocation density. The decrease in dislocation density is the main reason for joint softening. Continuous coarsening of T1/θ′ in heat affected zone (HAZ) and thermo-mechanically affected zone (TMAZ) weakens the strengthening effect. The development of precipitate-free zones and re-precipitation of TB primarily contribute to severe softening at the interface between HAZ and TMAZ on the advancing side (AS-HAZ-TMAZ). The serious softening behavior at the bottom of WNZ is attributed to the re-precipitation of TB and the complete dissolution of T1/θ′. Hardness profiles and tensile strength distribution of SSFSWed joint exhibit a “W” pattern, while a “U” pattern appears at the bottom. The hardness profile and tensile strength of WNZ are significantly poorer at 8–12 mm from the weld surface compared to those at 0–8 mm. Softening behavior also results from the higher fractions of the maximum Schmid factor (in 0.4–0.5) and Cube. From BM to WNZ, fracture mode transitions from quasi-cleavage fracture to plastic fracture.