Abstract: Studies on relationship of sulfur (S) source and poo1 in typica1 agro-ecosystems of Beijing suburb are carried out．The paper mainly deals with the characteristics of the system，effect of economy and farming style on S input and output．The results show that the source and pool of S in some subsystems can be changed at different periods．Based on the direction and rates of S flux into and from the system ，the pathways，complementary effects，profit and loss are determined quantitatively．For example，after a complete rotation，the outputs of S by Corn and wheat are 14．66 kg/hm² and 11．8kg／hm²，the inputs are 2．78kg／hm² and 9．73kg／hm² through roots and residues returned to soil，respectively．Taking account of all the pathways，46．1kg／hm² S is lost from and 37．2kg／hm² annually can be returned to the soil．Therefore，we can suggest that 8．9kg／hm² S per year should be replaced by S containing fertilizers to compensate S loss from the soil．