Cleanliness control is one of the most important processes for final optics assembly (FOA) in high-power laser systems. In this paper, an “Ultra Clean Manufacturing (UCM)” concept is put forward for the first time to satisfy the cleanliness requirements for FOA. First, the challenges and requirements for cleanliness in FOA are introduced and the concept of UCM is proposed. Significant surface cleaning technologies for mechanical parts and optics are then expounded. Finally, surface cleanliness control techniques, such as “gas knife” and “laser cleaning”, are developed as online cleaning methods for the surface cleanliness in FOA.