After binding to human serum albumin (HSA), bilirubin could undergo photo-isomerization and photo-induced cyclization process. The latter process would result the formation of a product named lumirubin. These photo induced behaviors are the fundamental of clinical therapy for neonatal jaundice. Previous studies have reported that the addition of long chain fatty acids is beneficial to the generation of lumirubin, yet no kinetic study have revealed the mechanism behind. In this study, how palmitic acid affects the photochemical reaction process of bilirubin in HSA is studied by using femtosecond transient absorption and fluorescence up-conversion techniques. With the addition of palmitic acid, the excited population of bilirubin prefers to return to its hot ground state (S0) through a 4 picosecond decay channel rather than the intrinsic ultrafast decay pathways (<1 picosecond). This effect prompts the Z-Z to E-Z isomerization at the S0 state and then further increase the production yield of lumirubin. This is the first time to characterization the promoting effect of long chain fatty acid in the process of phototherapy with femtosecond time resolution spectroscopy and the results can provide useful information to benefit the relevant clinical study.