Results of a 3-year long experiment on salinized fluvo-aquic soil in the Hexi Corridor region indicate that K application significantly raised the yield of potato, which increased with the rate of K application. But the production-increasing effect of K application decreased with the increase in application rate. According to regression statistical analysis, the optimal economical K application rate is 239.70 kg/hm². In this case the theoretical yield is 63.45 t/hm², the cost of K fertilizer, 599.25 yuan/hm², and the profit of the crop, 37,500 yuan/hm². The difference between treatments was significant and extremely significant based on LSD analysis. The crop response of potato to K application was negatively correlated to concentration of soil readily available K.