As the frame synchronization of the physical layer signal processing, timing synchronization (TS) is crucial for signal demodulation in vehicular communication systems (VCSs). However, existing TS methods primary rely on known training sequence to identify the first-arriving path, facing significant challenges in the scenarios with prominent Doppler and multipath interfere. Inspired by integrated sensing and communication (ISAC) technology, a sensing-aided TS method is proposed to address these challenges in VCSs. In this method, a detection threshold in the delay-Doppler (DD) domain is designed to extract echo paths. Based on the echo paths, we perform correlation matching between the Doppler prior information and the received communication signal, thereby further refining the sensing prior information for TS. With the assistance of Doppler prior information, the correctness of TS is improved by rectifying the local training sequence to compensate for the Doppler frequency shift of the received communication signal. In addition, an iterative method is developed to identify the first-arriving path by eliminating the impacts of paths stronger than the first one. Simulation results demonstrate that the proposed method significantly reduces TS error probability and exhibits its robustness against parameter variations.