Abstract: In order to study the effects of potassium fertilizer application and straw returning to soil on the yields of winter wheat and summer maize, crop potassium absorption and soil potassium balance, a field experiment was conducted under low soil fertility condition at the experiment site (located in Langfang city of Hebei Province) of National Laboratory of Soil Testing and Fertilizer Recommendation of Chinese Academy of Agricultural Sciences, North Plain of China from 2008 to 2011. The four treatments were designed with NP (no potassium fertilizer and no straw returning to soil), NPK (no straw returning to soil), NP+St (no potassium fertilizer but straw returning to soil) and NPK+St (potassium fertilizer application and straw returning to soil). The experimental design was the randomized complete block with three replications and each plot was 50 m2 (10 m5 m). Both in winter wheat and summer maize seasons, nitrogen and phosphorus application rates were N 180 kg/ha and P2O5 90 kg/ha, the potassium application rate was K2O 60 kg/ha in the treatments with potassium application, and the crop straw returning to field was managed by all winter wheat residue mulching and summer maize residue incorporation with soil. The results indicate the treatments of NPK, NP+St and NPK+St could significantly increase the crop yields with the same N and P application rates, the yield increment effects are NPK+St NPK NP+St NP. The different management practices have better yield effects in summer maize season than those in winter wheat season. The treatment of NPK+St has the highest crop yields and decreases the variation of crop yields in different years, which is benefit to sustain high crop yield. The potassium application and crop returning to soil have no significant effects on the potassium contents in grains or straws of the crops, however, which significantly improves the corps shoot absorption. Indies of potassium balance of the treatments of NPK+St, NPK and NP+S are 1.26, 0.42 and 0.92, respectively. Therefore, crops straw returning to soil or potassium fertilizer application can sustain soil potassium balance and soil potassium supply.