A plenty of theories on the origin of genetic codes have been proposed so far, yet all ignored its energetic relation to the biochemical system. Here, a new hypothesis is proposed, according to which ATP is at the origin of the primordial genetic code by its coevolution with the pristine biochemical system. This hypothesis aims to show how the genetic code was produced by photochemical reactions in a protocell that derived from a lipid vescile enclosing various life’s building blocks (e.g. nucleotides and peptides). At extant cell, ATP is the only energetic product of photosynthesis, and is at the energetic heart of the biochemical systems. ATP could energetically elongate chains of both polynucleotides and polypeptides, thus providing a bridge between these bio-polymers and eventually mediating cellular biochemical innovation from energy transformation to informatization. ATP was not the only one that could drive the formation of polynucleotides and polypeptides, but favored by precellular selection. The protocell innovated a photosynthesis system to produce ATP efficiently and regularly with the aids of proteins and RNA/DNA. The completion of the genetic code from RNA to DNA marked the dawn of cellular life operated by Darwinian evolution. The ATP hypothesis supports the photochemical origin of life, shedding light on the formation of both photosynthetic and biochemical systems, which remains largely unknown thus far.