An image sensor and an operation method thereof are provided. The image sensor includes a pixel circuit and a column readout circuit. The pixel circuit includes a pixel unit, a transfer transistor, a reset transistor, a readout transistor and a selection transistor. The column readout circuit includes a first circuit node and a second circuit node. A first terminal of the first reset transistor and a first terminal of the first readout transistor are coupled to a first circuit node, and a second terminal of the first select transistor is coupled to a second circuit node.