Chloroplast biogenesis during cold acclimation requires the precise coordination of tetrapyrrole biosynthesis (TPB) to prevent photo-oxidative damage while maintaining photosynthetic capacity. Here, we report that the H-subunit (CHLH, also named GENOMES UNCOUPLED5, GUN5) of Mg-chelatase is essential for the cold adaptation. We identified the gun5-1 mutant, (an allele from the previous gun5 mutant) which exhibits a severe chlorotic phenotype with a metabolic hindrance in chloroplasts under cold stress. We demonstrate that cold stress-incubated wild-type seedlings post-translationally stabilize CHLH/GUN5 in an assembled high-molecular-weight complex with GUN4, NADPH:protochlorophyllide oxidoreductase (POR), and glutamyl-tRNA reductase (GluTR), the rate-limiting enzyme of TPB pathway. In gun5-1, the disruption of this multienzymatic complex leads to the sequestration of the mutant GUN5 protein in the stroma and the accumulation of GluTR in inactive aggregates, resulting in a cessation of the TPB metabolic pathway. Our findings reveal a regulatory mechanism in which GUN5 goes beyond its catalytic role as a structural component and physically couples upstream and downstream enzymatic steps to optimize chlorophyll biosynthesis under cold stress.