A hybrid drive system for a vehicle comprises: a driving engine (100) having an output shaft (101); a motor generator (111); a planetary gear system having at least one staged planet gear set (105) mounted on a planet carrier (104) coupled to the output shaft (101) of the driving engine (100); the planetary gear system having a first sun gear (108) and a second sun gear (109); the motor-generator (111) being coupled to the first sun gear (108); the staged planet gear set (105) includes a first planet gear (106) and a second planet gear (107); the first planet gear (106) meshes with the first sun gear (108) and the second planet gear (107) meshes with the second sun gear (109); the ratio of working pitch diameter of first planet gear (106) to the second planet gear (107) being greater than one; the ratio of working pitch diameter of the second sun gear (109) to the first sun gear (108) being greater than one; a transmission (113) coupled between the second sun gear (109) and driven wheels (401); and a clutch (110) configured to selectively lock the planetary gear system, to bypass the planetary gear system.