Previous studies suggest that boreal summer intraseasonal variations along the subtropical westerly jet (SJ), featuring quasi-biweekly periodicity, frequently modulate downstream subseasonal variations over East Asia (EA). Based on subseasonal hindcasts from six dynamical models, this study discovered that the leading two-three-week prediction skills for surface air temperature (SAT) are improved significantly in summer when the SJ has strengthened intraseasonal signals, which are best demonstrated over the eastern Tibetan Plateau, Southwest Basin, and North China. The reasons are that the enhanced quasi-biweekly wave and the associated energy dispersion along the SJ cause more regular quasi-biweekly periodic variations of downstream SAT, which potentially increase regional predictability. This study suggests not only that intraseasonal variations along the SJ could provide a window of opportunity for achieving better subseasonal prediction over EA, but also that intraseasonal waves along the SJ are crucial for improving EA subseasonal prediction.