Light is a fundamental environmental cue that dynamically orchestrates plant growth and development through spatiotemporally regulated molecular networks. Among these, phosphorylation, a key post-translational modification, plays a crucial role in controlling the function, stability, subcellular localization, and protein–protein interactions of light signaling components. This review systematically examines phosphorylation-dependent regulatory events within the Arabidopsis light signaling cascade, focusing on its regulatory mechanisms, downstream functional consequences, and crosstalk with other signaling pathways. We underscore the pivotal role of phosphorylation in light signaling transduction, elucidating how the phosphorylation-decoding framework transduces light information into growth and developmental plasticity to modulate plant–environment interactions.