A TFT LCD array substrate and a manufacturing method thereof. The TFT LCD array substrate comprises a substrate. A gate line and a gate electrode that is formed integrally with the gate line are formed on the substrate. A first insulating layer and a semiconductor layer are formed sequentially on the gate line and the gate electrode. A second insulting layer covers sidewalls of the gate line and the gate electrode, the first insulating layer, and the semiconductor layer. An etching stop layer is formed on the semiconductor layer and exposes a part of the semiconductor layer on both sides of the etching stop layer. The TFT LCD of the present invention can be manufactured with a four-mask process.