Abstract: In this research, fall sweet potato of longshu3 were analysied by using the four factors orthogonal test on its plant density, N.application, K.application and applicaton methods of N. The results showed that the combination 3 was the most superior plan, its most average yield was 68.4kg with 3000 seedlings per 666.7m2, N application was 12kg, K application was 18kg and applicaton methods of N was more with early stage and few with later stage. The better plan was combination 7, its average yield was 65.2kg with 4000 seedlings per 666.7m2, N application was 8kg, K application was 18kg and applicaton methods of N was equivalent with division. The K application was the most important factor to the yield, the second factors were plant density and N application, and application methods of N. And the effects of different plant density on yield structure were analysied briefly in this paper.