6 N fertilizer application methods in tillering stage and booting stage were settled to study the effects of topdressing on population quality and yield formation of machine transplanted middleseason indica hybrid rice under the same fertilizer amount and percentage by field experiment.The results showed that the topdressing N fertilizer in tillering stage was more important than the booting stage in building the high yield and better population quality.N fertilizer application in 5 to 7 days and 12 ~ 15 days after the machine transplanting at the tillering stage could improve population earbearing tiller percent obviously,increase the LAI of booting stage and grainleaf ratio,improve the dry matter accumulation and distribution of the production after booting stage,increase the harvest index,optimize the structure of population and improve the machine in the plugin hybrid indica rice production.