A clock synchronizing circuit applied in a SMD block is provided. The clock synchronizing circuit includes a number of stages of clock synchronizing units. The clock synchronizing circuit can achieve the purpose of clock synchronizing by using a novel circuit design of the forward delay unit, the mirror control unit or the backward delay unit in each stage of clock synchronizing unit or by using a short-pulse generation circuit to generate a short pulse for triggering out an output clock of each stage of forward delay unit.