Field experiments were carried out to study the effects of different N rates on dynamic changes of root morphological characteristics and the yield and quality of lettuce cultivated in plastic greenhouse. The results showed that as compared with farmer habitual N usage, 40% reduction of N fertilizer input enhanced significantly lettuce root growth, mainly in increasing the root surface area, volume and total root length greatly; but the promotion of nitrogen fertilizer on root diameter was only observed in the late stages of growth. The highest yield and vitamin C contents were obtained in the treatment of 40% reduction of N fertilizer input. Furthermore, nitrate contents of lettuce increased with increase of N fertilizer input. With farmer habitual N usage, nitrate content of lettuce reached as high as to 1 291.9 mg/kg and exceeded the safety standard of safety and quality of agricultural products for root vegetables.