This invention relates to a plasma chemical vapor deposition microwave resonant cavity, which has a high focusing ability and can be flexibly configured. The resonant cavity is a rotary body formed by two isosceles triangles intersecting at the vertex angles with a Boolean union operation. The base angles of the two triangles are 50°˜75°. Between 2nλ˜(2n+0.5) λ, the base lengths of the two triangles are equal or have an nλ difference, where n is an integer and λ is the microwave wavelength. The distance between the centroids of the upper and the lower isosceles triangles is 0˜4/5λ. A strongly focused electric field can be formed in the cavity by adjusting the base lengths, base angles and centroid distance. Different dielectric windows, microwave coupling modes and gas inlet and outlet modes can be selected in the cavity to fit specific applications. The cavity has simple structures.