Abstract: To explore the promoting root mechanism of coupling of water and nitrogen, taking root growth of winter-spring cucumber and autumn-winter cucumber in solar greenhouse as a research object, the principle component analysis method was used to evaluate the root growth, the relationship between NO3--N content in different soils depth and distribution of root length density of root system in soil in effective root diameter was studied. The results showed that quality principle component could represent 97.27% primordial root information; the water management (irrigation quota was 300 m3/hm2 with irrigation scheduling of eight times for the winter-spring cucumber and five times for the autumn-winter cucumber) and suitable nitrogen application (240 kg/hm2 for the winter-spring cucumber and 50 kg/hm2 for the autumn-winter cucumber) situation could keep the soil NO3--N content in root zone appropriate for cucumber growth, increase yield of cucumber and reduce the wasting of water and nitrogen fertilizer, which the coupling mode of applying water and nitrogen with high quality and efficiency.