Two weak gluten wheat cultivars(Yangmai 13 and Ningmai 13) grown were used to investigate the effect of potassium fertilizer on the starch granules distribution and viscosity parameters in wheat grain. Four potassium rates of 0 kg/hm2(K0), 60 kg/hm2(K1), 120 kg/hm2(K2) and 180 kg/hm2(K3). The results showed that the volume and surface area percentage of B-type(≤10 μm) starch granules increased within the potassium rates from 0 to 180 kg/hm2 in weak gluten wheat. And the volume and surface area percentage of A-type(>10 μm) starch granules decreased within the potassium rates from 0 to 180 kg/hm2. It indicated that potassium fertilizer was favor of the formation of B-type starch granules. Increasing of the volume and surface area percentage of 2.5-10 starch granules were greater compared with≤2.5 μm starch granules in the B-type starch granules. Decreasing of the volume and surface area percentage of >20 starch granules in potassium treatment were greater compared with 10-20 μm starch granules in the A-type starch granules. Potassium fertilizer did not significantly affect the number percentage of both type starch granules in weak gluten wheat. The viscosity parameters with K1 and K2 treatments were higher than K0 and K3 treatments, which indicating that appropriate potassium application could increase starch viscosity parameters. Within the range from 0 to 120 kg/hm2 of potassium rate, the appropriate potassium fertilizer could increase the proportion of B-type starch granules and reduce the proportion of A-type starch granules, thus increase the viscosity parameters of starch in weak gluten wheat.