A device-to-device, D2D, synchronization method, wherein the method comprises: Acquiring, by first user equipment, synchronization control information of a serving cell, wherein the serving cell is located on a first transmission carrier frequency; Acquiring, by the first user equipment, an identity of a first synchronization signal from a first synchronization source that is obtained through searching on a second transmission carrier frequency, and signal quality of the first synchronization source; Determining, by the first user equipment according to the identity of the first synchronization signal, the signal quality of the first synchronization source, and the synchronization control information, whether to start to send a synchronization signal; and if it is determined to start to send a synchronization signal, sending, by the first user equipment, a second synchronization signal on a third transmission carrier frequency, wherein the second synchronization signal is used to instruct at least one second user equipment on the third transmission carrier frequency to perform synchronization according to the second synchronization signal, and a frequency of the third transmission carrier frequency is different from a frequency of the first transmission carrier frequency; wherein the synchronization control information comprises a physical cell identity, a sidelink synchronization signal identity corresponding to the physical cell identity and a first preset threshold; and the determining, by the first user equipment according to the identity information of the first synchronization signal, the signal quality of the first synchronization source, and the synchronization control information, whether to start to send a synchronization signal comprises: when the identity of the first synchronization signal is the same as the physical cell identity and the signal quality of the first synchronization source is less than the first preset threshold, determining, by the first user equipment, to start to send a synchronization signal; wherein after the determining, by the first user equipment, to start to send a synchronization signal, the method further comprises: using, by the first user equipment, a sidelink synchronization signal corresponding to the sidelink synchronization signal identity as the second synchronization signal. Acquiring, by first user equipment, synchronization control information of a serving cell, wherein the serving cell is located on a first transmission carrier frequency; Acquiring, by the first user equipment, an identity of a first synchronization signal from a first synchronization source that is obtained through searching on a second transmission carrier frequency, and signal quality of the first synchronization source; Determining, by the first user equipment according to the identity of the first synchronization signal, the signal quality of the first synchronization source, and the synchronization control information, whether to start to send a synchronization signal; and if it is determined to start to send a synchronization signal, sending, by the first user equipment, a second synchronization signal on a third transmission carrier frequency, wherein the second synchronization signal is used to instruct at least one second user equipment on the third transmission carrier frequency to perform synchronization according to the second synchronization signal, and a frequency of the third transmission carrier frequency is different from a frequency of the first transmission carrier frequency; wherein the synchronization control information comprises a physical cell identity, a sidelink synchronization signal identity corresponding to the physical cell identity and a first preset threshold; and the determining, by the first user equipment according to the identity information of the first synchronization signal, the signal quality of the first synchronization source, and the synchronization control information, whether to start to send a synchronization signal comprises: when the identity of the first synchronization signal is the same as the physical cell identity and the signal quality of the first synchronization source is less than the first preset threshold, determining, by the first user equipment, to start to send a synchronization signal; wherein after the determining, by the first user equipment, to start to send a synchronization signal, the method further comprises: using, by the first user equipment, a sidelink synchronization signal corresponding to the sidelink synchronization signal identity as the second synchronization signal. when the identity of the first synchronization signal is the same as the physical cell identity and the signal quality of the first synchronization source is less than the first preset threshold, determining, by the first user equipment, to start to send a synchronization signal; wherein after the determining, by the first user equipment, to start to send a synchronization signal, the method further comprises: using, by the first user equipment, a sidelink synchronization signal corresponding to the sidelink synchronization signal identity as the second synchronization signal.