Oligosaccharides could induce physiological responses of algae to defend attacks from pathogens. This study was designed to research the different responses of different growth stages of Pyropia haitanensis to agaro-oligosaccharide elicitor. Six growth stages including 50, 60, 80, 110 and 140 days P. haitanensis thallus and free-living conchocelis were selected to detect the release of hydrogen peroxide(H₂O₂) by(p-hydroxypheny1) acetic acid fluorometric method, expression of defense related genes(Phhsp70, Phrboh, Phsod, Phlox) by real-time quantitative RT-PCR, and volatiles changes by GC-MS after being elicited with agaro-oligosaccharides. The results showed that, without treatment with agaro-oligosaccharides, free-living conchocelis contains significantly higher levels of hydrogen peroxide and fatty-derived volatiles than those of the thallus. 100 μg/mL agaro-oligosaccharides can induce the quick outbreak of H₂O₂ of different growth stages of P. haitanensis, and thallus responded faster than conchocelis. Four defense-related genes were significantly up-regulated in conchocelis, such as Phlox was up-regulated 166.06±56.52 folds, while the genes of thallus at different growth stages exhibited a slight extent of up-regulation, moreover, the changes of these genes had variation trend at different stages. The composition and contents of volatile substances varied with different growth stages, but the 8-carbon volatiles predominated, such as 2-octenal, 5-octen-1-ol, 2-octen-1-ol and 1-octen-3-ol, etc. By using PCA method, the profiles of volatiles of thallus and conchocelis could be separated clearly, indicating an obvious difference in the profiles of volatiles between them. After being stimulated with agaro-oligosaccharides, some new volatiles in 80 d thallus and conchocelis were generated, and some 8-carbon volatiles were increased significantly. Overall, these results indicated that the different growth stages of P. haitanensis had different response modes to agaro-oligosaccharides, in which conchocelis responded most intensively.