The hydrolytic degradation behavior of stereocomplex polylactide (SCPLA) with distinct polymorphisms (α, αʹ, SC) and SC fractions (the ratio of SC crystallinity to total crystallinity) was first investigated in the present study. The evolution of three-phase crystalline compositions was quantified using modulated differential scanning calorimetry. Hydrolytic degradation commenced in the mobile amorphous fraction (MAF), while both the crystal (CF) and rigid amorphous fraction (RAF) began degradation on Day 3. By the 26th day, the MAF was consumed entirely, followed by the CF and RAF degraded at similar rates, primarily through the cleavage of chains with free ends on the folding surface of the remaining lamella. This work further discussed the influence of polymorphism on homochiral (HC) and SC crystal degradation during hydrolysis. Given racemic helices' participation in SC crystallization, SC-crystal lamellae may possess a higher number of amorphous chains with free ends than HC-crystals. This attribute could account for the faster degradation of HC-crystals in samples with a higher SC fraction.