We conducted hydroponic experiments in a plant growth chamber to study the effects of exogenous methyl jasmonate (0, 20, 40, 60, 80 and 100 μmol·L^-1 MeJA) on osmotic adjustment capacity in Jatropha curcas seedlings under salt stress (150 mmol·L^-1 NaCl). J. curcas seedlings under normal and salt stress conditions were treated with 0-100 μmol·L^-1 MeJA, and tissue vitality, MDA content, water content, water potential, the concentrations of compatible solutes proline, glycinebetaine, and soluble sugars in leaves of J. curcas seedlings, as well as the activities of the key enzymes Δ¹-pyrroline-5-carboxylate synthetase (P5CS), ornithine aminotransferase (OAT) of proline biosynthesis and the key enzyme betaine aldehyde dehydrogenase (BADH) of glycinebetaine biosynthesis, and the expression level of proline and glycinebetaine biosynthesis-related genes were measured. The results showed that: (1) exogenous MeJA treatment enhanced tissue vitality and water content, but it decreased water potential and MDA content in leaves of J. curcas seedlings under salt stress, and the optimum concentration of MeJA was 60 μmol·L^-1. (2) The application of different exogenous MeJA concentrations increased the contents of proline, glycinebetaine and total soluble sugars, and the 60 μmol·L^-1 concentration had the best effect. Treatment with 60 μmol·L^-1 MeJA significantly enhanced the accumulation of jasmonic acid, proline, and glycinebetaine in J. curcas seedlings under salt stress. (3) The MeJA treatment also clearly raised the activities of BADH, P5CS and OAT, and increased the expression level of the JcBADH, JcP5CS and JcOAT in leaves of J. curcas seedlings under salt stress. However, the MeJA treatment decreased the activity of proline dehydrogenase (ProDH), and inhibited JcBADH expression in leaves of J. curcas seedlings under salt stress. The results showed that exogenous MeJA promoted the accumulation of proline in J. curcas seedlings under salt stress by activating the glutamate and ornithine pathway of proline biosynthesis, especially the ornithine pathway, and inhibiting the proline degradation pathway. Meanwhile, MeJA also activated the biosynthesis of glycinebetaine in J. curcas seedlings. These results indicated that exogenous MeJA treatment can enhance salt tolerance and osmotic adjustment capacity in J. curcas seedlings under salt stress, and osmoregulation is a key factor in MeJA-induced salt tolerance.