A synchronous detector of an AC/DC vacuum circuit breaker with convenient operation and use comprises a power source, wherein the power source comprises a DC power source and an AC power source connected in parallel with each other; one end of the power source is connected with an AC/DC converter; a three-phase circuit breaker is connected in series inside a circuit; each phase of the three-phase circuit breaker is respectively connected in series with a contact point and an indicator light. The invention can select three 1.5V batteries or a 220V AC power supply (having a built-in transformer with the primary side equal to 220V and the secondary side equal to 4.5V). During use, a yellow line, a green line and a red line are correspondingly connected with both sides of the circuit breaker or a vacuum tube, and a vacuum circuit breaker is switched on step by step by means of a lever; an asynchronous phase can be intuitively detected through a shiny lamp bulb and then is adjusted till whether the three-phase contact of the vacuum tube is synchronous is judged; therefore, the synchronous detector brings great convenience to the checking operation of maintenance staff.