Abstract: The field trail was conducted to study the effect of seed yield and components by setting up different plant spacing of 5, 10, 15, 20, 25, 30cm with row spacing of 30cm under drip irrigation condition. Experimental data showed there were obvious influence in different density on pod rate and seed yield of alfalfa. The pod rate of 10cm was higher than others , up to 59.66%, but the seed yield was the lowest, only 572.83 kg.hm-2.The seed yield of 5cm was higher than others ,up to 1050.20kg.hm-2,which the next was 30cm,1002.15. kg.hm-2.The seed germination rate of 5 cm and 15 cm were 91.33%, which higher other treatments. Branches, pod number per inflorescence, seed number per pod, 1000-grain weight were more positively correlated with seed yield of alfalfa. With correlation coefficients of 0.54,0.523,0.474, 0.507, respectively. Comprehensive analysis the density has significant effect on factors of seed yield. The relatively high or low density can increase the seed yield, but the contribution rate that the different factor of seed yield is different in different density.