This application provides an operational amplifier and a start-up circuit of the operational amplifier. The start-up circuit has the advantages of having a simple structure and consuming less. The operational amplifier includes a multi-stage amplifier and a start-up circuit, where the start-up circuit includes a first start-up transistor and a second start-up transistor. A source of the first start-up transistor and a source of the second start-up transistor are connected to a tail bias node of a first-stage amplifier in the multi-stage amplifier, a gate of the first start-up transistor and a gate of the second start-up transistor are configured to connect to a first bias voltage Vb, and a drain of the first start-up transistor and a drain of the second start-up transistor are connected to input terminals of a second-stage or higher-stage amplifier.