The invention discloses a circuit breaker with a vacuum degree online monitoring function and a monitoring method for the circuit breaker. The circuit breaker comprises a vacuum sensor which is arranged in a vacuum arc extinguish chamber and is communicated with the interior of the vacuum arc extinguish chamber, wherein the vacuum sensor converts a vacuum pressure signal in the vacuum arc extinguish chamber into an ion flow signal through an ion column, and transmits the ion flow signal to a monitoring display device to display the vacuum degree state in the vacuum arc extinguish chamber in real time. The monitoring method comprises the following steps that: when an electric network is electrified, the vacuum sensor starts working to convert the vacuum pressure signal into the ion flow signal; a high-pressure signal is grounded and converted into a low-pressure safety signal through the ion column; a control processor processes the low-pressure safety signal to monitor the vacuum degree on line and display the gas pressure in the vacuum arc extinguish chamber; and the control processor compares the gas pressure with a preset standard figure to realize corresponding alarming or tripping actions. The circuit breaker is simple and convenient to use, a lot of manpower and material resources are saved, and the production cost is reduced.