The softening and foaming of lignin during pyrolysis are the main obstacles to the scale-up of lignin pyrolysis systems. In this study, the mechanism of borate crosslinking on lignin softening and pyrolysis process was investigated by combining in-situ physical and chemical techniques. First, the mobile protons and visco-elastic modulus during lignin pyrolysis were quantitatively analyzed by in-situ high temperature 1H NMR and rheology. It was found that the maximum content of mobile protons during the pyrolysis of raw lignin is around 90%. Hydroxyl crosslinking greatly inhibits the formation of mobile protons. The raw lignin produces a soft material from about 150 ℃ and starts to form a more rigid material from 200 ℃. At 210 ℃ ~ 330 ℃, mobile liquid-like intermediates are entrapped in the rigid char, forming “cavities”. With the addition of boron, the viscoelasticity tends to be stable and mainly elastic. Then, in-situ diffuse reflectance infrared Fourier transform spectroscopy and single photoionization time-of-flight mass spectrometry were used to characterize the evolution of functional groups and volatiles. It was found that the shielding of hydroxyl groups by borate inhibits the decomposition of lignin but improves the selectivity to phenols, and more oxygenated groups remain in the char