The invention discloses a rainbow printing ink fountain with a movable separation plate. The ink fountain comprises a contact inclined surface and circular arc side plates arranged at two sides of the contact inclined surface, wherein one end of the contact inclined surface is fixedly connected with a work platform in an inclined way, the ink fountain also comprises an ink fountain separation plate, and the ink fountain separation plate comprises a circular arc separation plate and a rubber pad. The invention also discloses an assembly method of the rainbow printing ink fountain with the movable separation plate. The ink fountain and the assembly method solve the problems that in the prior art, the ink color phase is easy to change, the printing precision is low, the flexibility is poor, the raw material waste is serious, the ink proportion is not adjustable, the cleaning is inconvenient, and the like. The ink fountain and the assembly method have the characteristics that the structure is simple, the operation is simple, the position of the ink fountain separation plate is adjustable, the isolation performance is good, the number of the ink fountain separation plates and the ink fountain separation plates in different thicknesses can be selected according to requirements, the mounting and the dismounting are convenient, the cleaning is convenient, the printing product quality is stable and reliable, and the like.