An array substrate, a manufacturing method thereof and a display device. The array substrate includes a substrate, a thin film transistor on the substrate, and including an active layer including a source region, a drain region and a channel region between the source region and the drain region; a heat dissipation layer disposed between the substrate and the drain region; and the orthographic projection of the heat dissipation layer on the substrate at least covers the orthographic projection of a part of the source region and a part of the drain region on the substrate. The manufacturing method is for the manufacturing of the array substrate. The array substrate can improve the sizes and uniformity of the crystal particles.