The source of Meltwater Pulse 1A (MWP-1A), the largest and most rapid global sea-level rise event during the last deglaciation, is important for understanding large ice sheet stability, global sea-level budget and climate change during last deglaciation. However, the source of this event is still debated, especially whether Antarctica can make a significant contribution to this event. Here we use a joint method of Monte Carlo linear regression and sea-level fingerprints constrained by six sites with sea-level observations through this interval to investigate the source and mangnitude of MWP-1A. By considering both uncertainties in age and depth, we found a significant 9.18 ± 3.42 m Antarctic contribution to MWP-1A. This result shows good agreement with some Antarctic palaeo proxies but it is inconsistent with most of Antarctic ice sheet (AIS) reconstructions dervived from glacio-isostatic adjustment (GIA) modelling studies, which either do not hold enough ice at Last Glacial Maximum (LGM) to make a significant contribution to MWP-1A, or do not show a rapid melting rate during MWP-1A. This is largely due to the poor understanding and discrete times of the current glacio-geological constraints on past ice extent. If Antarctica only contributed ~10 m to post-LGM sea-level rise as most GIA models predict, then the post-LGM sea-level rise is not balanced by the total amount of ice that melted since LGM. We quantify this ‘missing ice’ using a Monte Carlo simulation method, and show that there is 11.2 ± 12.3 m ice still unaccounted for. We also compare of observational (glaciological) constraints with GIA models, and conclude that Weddell Sea is the most problematic region in terms of fit between the observation and the models. This region could potentially hold some of the ‘missing ice’ and contribute significant sea-level rise during MWP-1A. More marine and terrestrial glacio-geological data, especially the constraints on ice grounding line at LGM, as well as more sea-level records across MWP-1A, especially the intermediate-field record in Central and South America region, are needed to test our hypothesis. In addition, for GIA models to accurately capture a transient AIS discharge during MWP-1A, it is necessary to consider the atmospheric and oceanic heat transfer, which plays an important role in MWP-1A AIS discharge.