The dark corner Mura produced in the long-term process of THO reliability is studied. Because the Mura is visible on the TFT film surface, the capacitance change in TFT side during the process of reliability is mainly simulated and analyzed, and it is determined that it is a kind of electrical defect caused by the change of Cst. The mechanism is that water continuously enters into the PVX2 film and Cst increases at the same times, resulting in the reduction of the charging voltage and gray scale of TFT-LCD pixels. After improving the compactness of PVX2 film by decreasing pressure and increasing Si/N ratio in the deposition process, the change of capacitance decreases from 19.6% to 05% in the simulation reliability test because of the better capability of water resistance, which successfully solves the problem, avoids the risk in the reliability test and improves the product quality.