Compared with conventional vehicle, axis-fixed gear parallel hybrid vehicle adds a motor which is usually installed between clutch and transmission. This will lead to increase transmission the input shaft inertia, synchronization inertia, speed, and rotor speed difference, which will make the conventional synchronizer difficult to meet the synchronization requires of the hybrid vehicle. This paper builds a mathematical model of synchronous process based on the actual working conditions of synchronizer and then analyses it in detail. Its focus is the influence of the increased synchronization inertia on synchronization time and heat capacity. In order to improve the gear-shift quality, some optimization measures aiming at synchronizer structure, lubrication, friction material, and active synchronization control are proposed in this paper. It does some forward-looking work for the synchronizer design of hybrid vehicle.