A cascade converter and an online monitoring method are provided. The capacitor parameters of a bus capacitor of a cascade unit is acquired according to the voltage across the bus capacitor of the first target cascade unit, the cascade side current of the single-phase cascade module and a switching signal from the first target cascade unit. In such way, it is not necessary to increase the number of the current sensors. The capacitor parameters of the bus capacitor herein include the equivalent series resistance and the capacitance. Since the capacitor parameters of the bus capacitor are monitored constantly, the aging degree of the bus capacitor can be checked at any time.