The softening behavior of lignin has a significant but often overlooked impact on its pyrolysis process. In this study, the mechanism of lignin softening and pyrolysis was investigated with in-situ high temperature 1H NMR and rheology. The results indicated that lignin underwent stages of softening, apparent solidification, substantial solidification, and carbonization. During the softening stage (180~220 ◦C), the weak ether bonds in the rigid lignin chain were broken to produce a large quantity of mobile fragments, and dehydration and decarboxylation reactions began, the matrix flow resistance was significantly reduced. In the apparent solidification stage (220~360 ◦C), the internal mobile phase of the matrix expanded to nearly 100%, but the surface mobile species cross-linked, and the flow resistance increased. In the substantial solidification stage (360~470 ◦C), the crosslinking and polycondensation of the mobile fragments dominated, causing the mobile phase to shrink. The secondary cracking of mobile fragments is the primary route to form gas-phase products. In the carbonization stage (>470 ◦C), no mobile phase remained, the matrix further deoxidized, polycondensation to form rigid char, and only a small amount of gas was released. Lignin with high molecular weight and high C–C bond content has a slower softening rate and narrower mobile phase distribution, thus hinders the formation of gas-phase products.