The present disclosure provides a thin film transistor, an array substrate, a display panel and a display device. The thin film transistor comprises a gate layer, a source and a drain located on the gate layer, and an active layer located on the source and the drain. The active layer is electrically connected to the source and the drain. The active layer comprises two sides arranged in parallel, and each side forms an acute angle of 45° with a face of the drain facing the source.