The acid strength of metal-organic frameworks plays a key role in their catalytic performance such as activity and selectivity during catalytic reactions. Solid-state nuclear magnetic resonance in combination with probe molecules including 2-13 C-acetone and pyridine-d5 was employed to characterize the acid strength of UiO-66-X (X = -H, -2COOH, -SO3 H). It was found that after introduction of the functional groups, the acid strength of UiO-66-2COOH and UiO-66-SO3 H is considerably enhanced compared with that of parent UiO-66, with that of the former being similar to that of zeolite H-ZSM-5, and with that of the latter being slightly stronger than that of the former. Even though the acid density can efficiently be modified through changing the relative ratio in multivariate functionalized UiO-66-X, no significant alternation for the acid strength could be discerned in the MTV-UiO-66-X compared with acidic same-link counterpart. Theoretical calculations were employed to further confirm the acid strength of UiO-66-SO3 H and UiO-66-2COOH.