The invention relates to a layer-one synchronization method of a forward transmission interface based on a user-defined Ethernet frame, and provides a layer-one synchronization system, which comprises a HUB processing unit and a pRRU processing unit, and comprises the following steps: taking the HUB processing unit as a main part, initiating a layer-one synchronization process, taking the pRRU processing unit as a slave part, and after receiving a layer-one message, carrying out synchronization detection to complete layer-one synchronization. According to the method, the first-layer synchronization process is simplified, bottom layer processing resources are saved, the resource utilization rate is effectively improved, the method is applied to an extended small base station system, logic resource loss is reduced, the first-layer synchronization process is optimized according to the customized CPRI framing, control over cost and power consumption is facilitated, and the implementation mode of a remote unit is optimized.