Taking Muguandao Reservoir, a proposed bay reservoir, as the research object, the critical wind speed of sudden mixing in the bay reservoir was discussed from the point of view that the internal falsehood caused the mixing of salt water in the bottom boundary layer and fresh water in the surface layer. The critical wind speed of sudden mixing of Muguandao reservoir was calculated by using Shintanis improved effective Wadburn number, combined with the topography and stratified position of Muguandao Reservoir. When the wind direction is NNW, the maximum blowing distance is 3103.45m on the water surface, and the wind speed is more than 20.5m/s at 10 m above the lake surface, the water of Muguandao Reservoir suddenly mixes. Under the same conditions, the critical wind speed of sudden mixing in the cuboid reservoir is 21.7m/s. When the wind direction is SW, the maximum blowing distance is 3620.69m on the water surface, the critical wind speed of sudden mixing in the reservoir is 22.7 m/s. Under the same conditions, the critical wind speed of sudden mixing in the cuboid reservoir is 19.6m/s. When the wind direction is SW(from sea to land), the sudden mixing in reservoir is more difficult, although the maximum surface blow distance of Muguandao Reservoir is larger than that of NNW(from land to sea). And in these two different wind directions, the critical wind speed of the reservoir is opposite to the critical wind speed of the cuboid reservoir.