The utility model discloses a single-planet-carrier planet variable speed transmission device with a large transmission ratio. An inner gear (2) is installed in a box body (1), and a planet carrier (3) is installed in the box body (1) and a box cover (12) through a planet carrier bearing (10); double rows of planet gears (4) are installed in the planet carrier (3) through a planet gear bearing (6) and a planet gear shaft (5), double rows of solar gears (7) are installed in the box cover (12) and the planet carrier (3) through a solar gear shaft (8) and a solar gear shaft bearing (9), a connecting sleeve (14) is connected with the double rows of solar gears (7), a gear shifting sliding block (15) is located in an annular groove (13) in the connecting sleeve (14), and bearing end covers are installed at the outer end of the solar gear shaft bearing (9) and at the outer end of the planet carrier bearing (10). The double rows of planet gears and the double rows of solar gears are arranged, gears in each row in the double rows of solar gears are meshed with corresponding gears in the double rows of planet gears respectively to achieve different transmission ratios, and the tooth numbers of the gears in each row in the double rows of planet gears are different to achieve the larger transmission ratio.