Altitudinal treeline is typically considered as being sensitive to climate change, and treelines worldwide have been responding to climate warming by increasing in elevation and stand density. However, little is known how upward shift and tree densification within treelines are interrelated. Studies of these two ecological processes at regional and local scale have yielded heterogeneous results, showing that either some treelines displayed tree densification without significant advance in their positions or inversely some treelines have shifted their positions upward without any noticeable densification. To explore this variation, we here quantified the difference between the rate of altitudinal upward shift and the rate of tree densification within treelines worldwide and identified its global drivers. The mismatch between upslope shift and tree densification has been observed widely within over 63% of the global treelines, which was primarily driven by temperature change, showing that compared with the rate of tree densification, the rate of treeline shift tended to decrease when accompanied by the increasing rate of warming. Thus, our findings revealed that the accelerating climate warming has enlarged the mismatch between the rate of upward shift and the rate of tree densification and thus widened the lagging gap of global treeline shift relative to densification.