A series/parallel hybrid electric drive unit for vehicle comprises an engine (1), a main traction motor (2), an integrated starter-generator (3), a differential (4), a first shaft (main shaft) (5), a first stage decelerating device (9), a second stage decelerating device (10), a first clutch (6), a second clutch (7) and a synchronizer (8). The synchronizer (8) is slidably arranged on the first shaft (5) which is connected to the first stage decelerating device (9) or the second stage decelerating device (10) by the synchronizer (8), respectively. The hybrid electric drive unit of the invention has a compactly-arranged internal structure and an efficient and appropriate internal connection, and a switch between connection and disconnection of respective hybrid power sources and wheels and a shifting among operating modes and gear positions of the hybrid drive system can be realized.