The invention discloses an image sensor and a forming method thereof. The image sensor comprises a pixel array, and the pixel array comprises a plurality of pixel units which are arranged in an array mode. Each pixel unit comprises a semiconductor substrate, a photodiode, a floating diffusion region, a transfer transistor and a source-follower transistor, wherein the photodiode is placed in the semiconductor substrate, the floating diffusion region is placed in the semiconductor substrate, the transfer transistor comprises a source electrode and a drain electrode which are placed in the semiconductor substrate, the source electrode and the drain electrode are electrically connected with a photovoltaic conversion element and the floating diffusion region respectively, the source-follower transistor comprises a grid electrode placed on the semiconductor substrate, the grid electrode is electrically connected with the floating diffusion region, a channel region of the source-follower transistor is of a cross beam structure, the cross beam structure is provided with a top face and two side faces, and the grid electrode of the source-follower transistor covers at least one of the top face and the two side faces. The performance of the image sensor is improved, and cost is lowered.