The high temperature application of polylactide (PLA) has been greatly limited by its low heat deflection temperature (HDT). As a slow crystallizing material, the HDT of amorphous PLA is mainly determined by its low T
g value, near 55°C. In this work, PLA was chemically crosslinked, and the HDT of the crosslinked PLA was evaluated from two aspects—crystallization ability improvement and chain mobility limitation at T
g of the PLA. The crystallization rate is improved by crosslinking because of the increased nucleation efficiency, and reached the highest level at moderate crosslinking. The final crystallinity is, however, still far lower than the crystallinity threshold for enhancing HDT of PLA. More importantly, T
g of the PLA is not influenced by the crosslinking, even the average molecular weight between the crosslink points reaches as low as 1654 g/mol. The experimental research and theoretical calculations indicate that, to improve the HDT of the PLA, conformational restriction on the PLA chains should be focused on the segments with less than, at least, 14 repeating units. The fundamental reason why the HDT of PLA cannot be improved by crosslinking is investigated to guide the future structure design for high HDT PLA products.