Light intensity plays a critical role in determining spike fertility of wheat (Triticum aestivum L.) by influencing floret development. However, whether a light signal directly perceived by the juvenile spike is essential for spike fertility remains unknown. To address this, we conducted two experiments imposing light blocking (LB) treatments from jointing to either anthesis (LB_J-A) or booting (LB_J-B) as well as a control (CT). LB was imposed through an innovative procedure employing aluminum foil. We found that LB_J-A did not affect floret primordia initiation, only increased distal floret mortality, and drastically impaired pollen viability in the anthers, resulting in a slight reduction in the number of competent florets but a massive sterility of all florets across all spikelets compared to CT, and LB_J-B exhibited results closer to the CT. For instance, under field conditions, the number of fertile florets was 42.92 in CT and 36.10 in LB_J-B, whereas no fertile florets (0) were observed in LB_J-A. While re-exposing juvenile spikes to light at booting restored fertility, highlighting that light signal perceived by juvenile spike from booting to anthesis played a crucial role for floret fertility. Furthermore, a strong correlation was observed between the number of fertile florets and the number of floret primordia with green anthers, suggesting that light blocking may impair fertility by preventing anther greening. These novel findings provide evidence that light signal directly perceived by juvenile spikes qualitatively may affect spike fertility possibly through affecting anther greening and inducing sterile pollen formation.