The Madden-Julian oscillation (MJO) has remarkable impacts on global weather and climate systems. Understanding its changes under a warming climate provides insights into how MJO-related phenomena may change accordingly. This study examines the future changes in MJO projected by 23 Coupled Model Intercomparison Project Phase 6 (CMIP6) models that produce a realistic MJO propagation in their historical runs. Results from the multi-model mean show a ~17% increase in MJO precipitation amplitude, a ~9% increase in propagation speed, a ~2-day decrease in MJO period, and a ~5° eastward extension. Analysis of the lower tropospheric moisture budget suggests the dominant role of an increased meridional advection of mean moisture caused by the steepening of mean moisture over the Indo-Pacific warm pool in a warming climate. This leads to a stronger positive moisture tendency to the east of MJO convection, and hence a more eastward MJO propagation with strengthened amplitude and faster speed.