Field experiments were conducted to determine the interactive effect of sulfur (S) and nitrogen (N) applications on seed yield from Psoralea corylifolia L. Six treatments were tested: $T_1$ = control (without manure and fertilizers), $T_2$ = manure @ 9 kg $plot^{-1};(10;t;ha^{-1}$), $T_3=S_0N_{20}K_{40}P_{40}$, $T_4=S_{20}N_{20}K_{40}P_{40}$, $T_5=S_{20+20}N_{20}K_{40}P_{40}$, and $T_6=S_{20+20}N_{20+20}K_{40}P_{40}$. Activities of nitrate reductase (NR) and ATP-sulfurylase in the leaves were measured at various phenological stages. These two enzymes catalyze the rate-limiting steps in the respective assimilatory pathways for nitrate and sulfate. Enzyme activity was strongly correlated with seed yield, with the greatest performance being achieved with treatment $T_5$. This might be attributed to the optimization of leaf soluble protein and photosynthetic rate, both of which are influenced by S and N assimilation.