In this study, a field experiment was conducted under straw returning into the field, which included no potassium fertilizer (CK), high-nitrogen/low-potassium compound fertilizers containing nitrogen, phosphorus and potassium nutrients of 18%, 12% and 10% (F), and F with potassium fertilizer (F+K) treatments. Then, the changes in yield, yield components, nitrogen, phosphorus, and potassium accumulations, and potassium fertilizer utilization rate of early and late rice under different treatments were analyzed. The results showed that compared with CK, the early rice yields under F and F+K were increased by 4.36% and 9.13%, the late rice yields were increased by 4.09% and 13.89%, and the total rice yields were increased by 4.22% and 11.65%, respectively. At the same time, the early and late rice yields under F+K were higher than those under F by 4.57% and 9.41%, respectively. The analysis of yield components showed that F+K mainly achieved rice yield improvement by increasing the number of spikes per plant in the early rice season, the number of spikes per plant and thousand grain weight in the late rice season. Because F+K increased the potassium contents in grains and straws, the accumulations of nitrogen and potassium in the early rice season under F+K were higher than those under CK by 12.30% and 22.40%, respectively, while the accumulations of nitrogen, phosphorus, and potassium in the late rice season under F+K were higher than those under CK by 11.89%, 21.62%, and 26.59%, respectively. The results of both seasons also showed that the accumulations of nitrogen, phosphorus, and potassium under F+K were increased than those under CK by 12.07%, 22.45%, and 24.54%, respectively, and they were improved by 9.34%, 8.56%, and 11.28% higher than those under F. Due to the large amount of potassium brought in by straw return, there was no significant difference for potassium use efficiency between F+K and F, but the variability of potassium use efficiency under F+K was significantly reduced than that under F. Therefore, it is recommended that high-nitrogen/low-potassium type compound fertilizer should be combined with potassium fertilizer in southern double-cropping rice areas to achieve the goal of rice yield and efficiency improvement under straw return conditions.