In order to clarify the differences in growth and nitrogen metabolism of rice seedlings of different ages under soda salinealkaline stress, Changbai 9 was used as test material, rice seedlings of different ages, L1 (3-leaf seedling age), L2 (4-leaf seedling age) and L3 (5-leaf seedling age), were stressed with 50 mmol/L mixed saline alkali, and the physiological response, material accumulation and nitrogen absorption characteristics of the rice seedlings were studied after soda salinealkali stress (0-48 h). The results showed that soda salinealkali stress significantly increased the content of proline and soluble sugar in rice seedlings, the response time of the 3-leaf seedlings was earlier and the increase was greater, and with the increase of age, the osmotic adjustment ability became weaker; Soda saline-alkali stress significantly increased Na+ accumulation and Na+/K+ in leaves and roots of rice seedlings of all ages, the peak value of 3-leaf seedlings was higher, and the ion balance was gradually enhanced with the growth of rice seedlings. From the perspective of nitrogen metabolism, soda salinealkali stress significantly inhibited nitrogen uptake and transformation in different seedling ages, the activities of NR and GS in leaves and roots were reduced, and nitrogen accumulation in shoots and dry matter accumulation in plants were inhibited. The decrease of NR and GS activities in the 5-leaf seedlings was significantly less than that of other seedlings, and the GDH metabolic pathway was enhanced by slightly upregulating the GDH activity of leaves, so as to adapt to the effects of salinealkali stress on nitrogen uptake. The effects of 5leaf seedlings on shoot dry weight, root dry weight and nitrogen accumulation were also significantly less than those of other seedling ages. The above results show that transplanting rice in soda salinealkaline land to select ageing seedlings with large leaves is beneficial to enhance seedling resistance and ensure more efficient nitrogen absorption and material accumulation.