It has been reported that the augmented vertical axis turbine (VAT) has a better operating performance. Comparing to the duct-type diffuser, a guide-vane type augmentation proposed in this study has a simpler outline and a better adaptability to various directions of the incident water flow. A 2D numerical model based on the computational fluid dynamics software ANSYS-Fluent was established and validated by the experimental data. It was found that the guide vanes could narrow the flow path together with the VAT rotor and increase the flow velocity around the blades. The fluctuation of the instantaneous torque output was significantly reduced by using the guide-vane stator. The numerical results indicate that four blades setup is suitable for the stator and the chord length of the guide vane should be equal to that of the rotor blade. The gap between the stator and rotor is suggested to set at a quarter to the chord length of the rotor blades. The non-zero pitch angle of the guide vane is found to have negative effects on the torque and power output. The averaged power and torque coefficients for three non-zero directional angles of the incident flow are approximately 30% lower than that for the zero-directional angle.