The Mongolian Plateau (MP), a typical arid and semi-arid region, hosts fragile ecosystems that are highly sensitive to variations in precipitation. Within a summer season, precipitation in the MP exhibits significant 10-30-day variability, accounting for similar to 40% of the total precipitation variability. This study investigates the characteristics of quasi-biweekly precipitation in the MP and evaluates its representation in 20 CMIP6 climate models. Surprisingly, all models underestimate the intensity of the quasi-biweekly precipitation (only 35-80%). Further analysis shows that this deficiency mainly results from the insufficiency of dynamic conditions associated with ascending motion rather than the thermodynamic conditions related to moisture. Diagnosis of the scale-decomposed moisture budget equation reveals that the anomalous water vapor in the MP mostly comes from horizontal moisture advection, in which both anomalous southeasterlies transporting the background moisture and background westerlies transporting the moisture perturbations into the MP play positive roles. CMIP6 models reasonably capture these moisture processes, but they poorly simulate the mid-latitude wave-train system that is critical for generating ascending motion. The pronounced positive vorticity advection observed on the lee-side of the Altai Mountains is not captured in the CMIP6 simulations. Consequently, compared to the ascending motion appearing over the entire MP in the observations, the simulated upward motion is only present in the eastern plateau. The negative biases of the circulation anomalies greatly suppress the quasi-biweekly rainfall intensity, highlighting dynamical deficiencies in current climate models.