A soil column incubation experiment was conducted in this study to investigate the effect of NH4+ on transformation and movement of K+ in ferti-sphere of paddy soil. The results showed that movement of K+ increased with K dose. In presence of same amount of NH4+, as K dose increased by 2.4 times, the water-extractable K (WE-K), Exchangeable K (E-K) and non-exchangeable K (NE-K) in the nearest part of ferti-sphere significantly increased by 2.1, 1.7 and 0.3 times, respectively. While the K diffusion distance increased by 16.4% (12 mm) as K dose increased. The results indicated that fixation of K in ferti-sphere is somewhat limited; more portion of K remained in labile form as K dose increased. Co-application of NH4+ and K significantly increased both WE-K in the nearest part of ferti-sphere and the K diffusion distance as compared with K solely applied treatment. Co-application of NH4+ significantly increased E-K in the transition area from ferti-sphere to bulk soil and reduced NE-K in all the soil samples that NH4+ possibly reached. Co-application of NH4+ significantly restrained transformation of added K into NE-K in ferti-sphere, and the effect declined as the distance from ferti-sphere increased. Different models were compared to describe the change of K concentration from ferti-sphere to bulk soil, for both WE-K and E-K, the anti-S-shape curve y = a / [1 + b exp (c × x)] can be well fitted.