The invention discloses a double-main-shaft type ultraprecise fly cutting milling machine, and relates to a milling machine which solves the problems of low rigidity and low machining efficiency of a main shaft system of the existing KDP (potassium dihydrogen phosphate) ultraprecise machining machine tool. The milling machine comprises a gantry type machine tool body, a longitudinal linear guide rail system, a left horizontal electric main shaft system, a right horizontal electric main shaft system and an air vibration isolation supporting system, wherein the longitudinal linear guide rail system is arranged at the middle part of the gantry type machine tool body and is fixed on a base; and the left horizontal electric main shaft system is arranged on a left vertical column, the right horizontal electric main shaft system is arranged on the right vertical column, the longitudinal linear guide rail system is arranged between the left horizontal electric main shaft system and the right horizontal electric main shaft system, a left vacuum suction plate are oppositely arranged and the left horizontal electric main shaft system are oppositely arranged, and a right vacuum suction plate and the right horizontal electric main shaft system are oppositely arranged. According to the double-main-shaft type ultraprecise fly cutting milling machine disclosed by the invention, high-precision linear feeding motion, rotation motion of a cutterhead and synchronous cutting of double workpieces are realized, and the guide rail and the main shaft have the advantages of high precision and rigidity by utilizing a hydrostatic pressure control manner.