Abstract: In order to improve the thermal-hydraulic performance of mixing vane spacer in pressurized water reactor (PWR) under the condition of the subcooled boiling, the effectiveness of the subcooled boiling numerical model currently used was validated. Based on the validated numerical model, a series of two-phase simulations of subchannels with mixing vane spacer grids for different vane slope angles were performed under different working conditions. The influences of mixing vane and its slope angle on subcooled boiling flow, heat transfer and void fraction distribution were investigated. The results show that the mixing vane spacer grid not only causes more pressure loss, but also enhances heat transfer and decreases the void fraction in the near-wall zones. The influence is more obvious with the increase of the slope angle within a certain range. Relatively higher void fraction in rod bundle can lead to more pressure drop, lower mixing performance and lower heat transfer efficiency. While the slope angle of mixing vane reaches to 25°, continuing to increase the angle can not decrease void fraction in nearwall zones and increase heat transfer efficiency further, but cause more pressure loss. Therefore, the suggested slope angle of mixing vane is about 25°.