Abstract: In order to research the relativity between main traits of leaves and protein and yield of soybean under different nitrogen treatment, the experiment took ‘Hefeng No.55’, a variety of soybean, as the experimental material, established four different N treatments and measured its leaf area index and the content of chlorophyll and protein content at five different growing period after flowering. The results of the test showed that: the main leaf traits had the closely positive relativity with yield in R1-R7. The relativity leaf area index of every N fertilizer treatments at every periods, and yields attained remarkable even more remarkable level. The relativity between leaf area index and yield reached remarkable level at R1 period and R5 period and the average of all growing period, and the relativity between leaf area index and yield at R6 period R7 period reached more remarkable level. The relativity between yield and the content of chlorophyll reached remarkable level at the period of R5 and R6 and the average of all growing period; the relativity between of yield and protein reached negative correlation, and reached remarkable negative correlation at late growth period.