The QuantumATK software package based on density functional theory and non-equilibrium Green's function is used to predict the performance of a 5.1 nm in-plane contact double-gate WSeTe Schottky barrier SBFET with 1 T-WTe2 as electrodes and Janus WSeTe as channel. It is found that the performance of 5.1 nm WSeTe SBFET meets the prediction parameters of ITRS(2013 version) about high performanc device for the production year 2028, but Te (Se) vacancy defect at the channel near the source will degrade its performance. How to repair the performance of the WSeTe SBFET with Te (Se) vacancy defect at the channel near the source? The effects of O, Cl and N passivation of Te (Se) vacancy defects on the performance of WSeTe SBFET were discussed. The results indicate that O Passivation can repair and even improve the performance of WSeTe SBFET by repairing the DOS changes of monolayer WSeTe induced by Te (Se) vacancy defects.