The present disclosure discloses a trench gate semiconductor device, wherein a trench gate includes a trench formed in a semiconductor substrate, and a gate oxide layer formed on a bottom surface and a side surface of the trench; the gate oxide layer is formed by stacking a first oxide layer and a second oxide layer; the first oxide layer is a furnace tube thermal oxide layer; the second oxide layer is a PECVD oxide layer; the gate oxide layer has a thermally densified structure processed by means of RTA. The present disclosure also discloses a method for manufacturing a trench gate semiconductor device. The present disclosure can increase BVGSS of the device, without affecting the threshold voltage of the device, with simple processes and low costs.