The invention discloses a single-arm type milling and turning composite double-rotation workbench. The single-arm type milling and turning composite double-rotation workbench comprises a workbench body, a horizontally-arranged first rotation mechanism and a second rotation mechanism perpendicular to the first rotation mechanism. The second rotation mechanism is installed on the first rotation mechanism in an auxiliary manner, and a series connection integrated structure is formed by the second rotation mechanism and the first rotation mechanism. The upper end face of the second rotation mechanism is covered with the workbench body. Each of the first rotation mechanism and the second rotation mechanism of the single-arm type milling and turning composite double-rotation workbench is internally provided with a direct drive motor assembly, each of the first rotation mechanism and the second rotation mechanism is provided with a hydraulic rotation locking mechanism, and therefore the first rotation mechanism can be locked at any position in the rotation stroke. A front bearing set and a rear bearing set are adopted in the second rotation mechanism for supporting, the high-speed rotation direct drive motor assembly is arranged in the part between bearings, and therefore the second rotation mechanism can continuously rotate at the speed of 1,000 r/min, and a milling rotation shaft is converted into a turning main shaft through cooperation of the second rotation mechanism and the first rotation mechanism. The single-arm type milling and turning composite double-rotation workbench is compact in structure, high in rotation shaft precision, good in dynamic performance and capable of achieving conversion of the milling function and the turning function.