Abstract: In the relay-cropping system of “wheat/maize/soybean”, the soybean cultivar, Gongxuan 1, was used to study the effects of phosphorus and potassium fertilization on potassium accumulation and utilization efficiency of soybean under the relay-cropping system. The results show that the potassium accumulation of the soybean during the whole growth period can be described as an “S” growth curve. Total K accumulation amount and K contents in roots, stems, leaves and legumes are all increased with the increase of K application amount, while the K accumulation amount and the K contents are increased at first and then decreased with the increase of P application amount, and the K accumulation amount and the K contents are all highest under the P2K3 (P2O5 17.0 kg/ha, K2O 112.5 kg/ha) treatment, which are 18.79%-58.33% higher than those of the P0K0 (without phosphorus and potassium application) treatment. The change of K accumulation rate in the whole period shows a single peak curve, and reaches to its largest value at about 90 days after the emergence. The changes of K production efficiency, uptake and utilization efficiency and K agronomy efficiency are similar to that of the K accumulation rate, while the change of the harvest index is decreased at first and then increased with the increase of the phosphorus application. The appropriate phosphorus and potassium application could improve the K utilization efficiency, the optimal treatment is the P2K1 (P2O5 17.0 kg/ha, K2O 37.5 kg/ha).