The invention discloses a nonstandard milling cutter for forming a continuous three-dimensional curved surface and a method for machining by means of the nonstandard milling cutter. The nonstandard milling cutter comprises a milling cutter body and a cutter bit arranged at one end of the milling cutter body. The cutter bit is provided with a cutting blade and a chip removal groove. The chip removal groove is gradually transited to the outer wall of the milling cutter body from the cutting blade. The end face of the cutter bit and the three-dimensional curved surface to be machined match. According to the nonstandard milling cutter for forming the continuous three-dimensional curved surface and the method for machining by means of the nonstandard milling cutter, the end face of the cutter bit and the three-dimensional curved surface to be machined match so that the three-dimensional curved surface can be formed at a time; on the basis of guaranteeing the size accuracy of machined grooves, machining efficiency can be greatly improved; and the cutter is low in cost and convenient to replace. The nonstandard milling cutter for forming the continuous three-dimensional curved surface and the method for machining by means of the nonstandard milling cutter are suitable for field of machining cutters of nuclear equipment.