The invention discloses a method for cultivating mechanized transplanted seedlings of rice by use of an organic medium production line in a gutter-connected plastic greenhouse. The method comprises the following steps of 1), arrangement of cultivating seedling materials, namely a medium, a seedling disk, a dome-shaped steel-structure plastic film gutter-connected glasshouse facility, and a finishing seedbed; 2), seed treatment, namely choosing seeds, soaking the seeds and accelerating germination; 3), medium treatment and disk loading, namely firstly loading a bottom-layer medium, then seeding, finally covering the seeds with the medium and watering; 4) seeding and bud promotion, namely seeding dry rice and bud rice in each disk, stacking the disks and promoting bud, moisturizing, warming, arranging the disks, and performing atomizing spray, wherein the disk stacking height is 20 layers; and 5) seedling stage management, namely attentively managing the temperature and moisture and controlling diseases. The method has the advantages of good technical effect, obvious social benefit and prominent economical and ecological benefits and the like, and the method has preferable popularization prospect.