Abstract: A planetary gear speed reducer includes a gear bracket having a center hole and first, second and third locating portions arranged in parallel along the center hole, a sun gear mounted in the center hole of the gear bracket, first gears rotatably mounted in between the first and second locating portions of the gear bracket and meshed with the sun gear, second gears rotatably mounted in between the second and third locating portions of the gear bracket and meshed with the sun gear, and two annular gears respectively meshed with the first gears and the second gears. Enabling the two annular gears to mesh with the first and second gears respectively avoids installation inconvenience due to phase difference between the first and second gears and eliminates interference during rotation of the two annular gears. By means of changing the number of teeth of each gear, the planetary gear speed reducer can provide a low, medium or high reduction ratio.