A pot culture experiment of nitrogen forms (NO3-:NH4+) with four ratios (100:0, 75:25, 50:50, and 25:75) in nutrient solutions was conducted to examine the effect of nitrogen forms on the growth and polyamines contents of developing seeds of vegetable soybean [Glycine max (L.) Merr. cv. Li-xiang 95-1]. Results showed that the best plant growth vigor was observed in NO3- (75%), and then in NH4+ (50%). However, the fresh and dry weight of biomass decreased when a high concentration of either NO3- (100%) or NH4+ (75%) was the primary nitrogen source in the nutrient solution. The numbers of flowers and pods in NH4+ (75%) were significantly decreased compared with other ammonium-nitrate ratios in which the numbers of flowers and pods were not influenced by nitrogen forms. During the development of seeds, levels of free Put and Spd decreased, and the decrease extents were more marked in 100:0 and 25:75 (NO3-:NH4+), but the change of free Spm was opposite. Levels of conjugated and bound polyamines in all nitrogen forms increased, but the increases in 75:25 and 50:50 (NO3-:NH4+) were not so obvious as in 100:0 and 25:75 (NO3-:NH4+) treatments. The possible roles of polyamines in the adaptive mechanism of vegetable soybean seeds to different nitrogen forms were discussed.