With a crystallizer（diameter is 1 200 mm, height of cylinder was 1 200 mm, heat transfer area is 5.8 m2） in pharmaceutical factory as an example, the design of crystallizer structure and agitation system were analyzed. The conventional jacket was used as heat exchanger structure, and the anchor was chosen as the stirring impeller. The rotational speed of anchor should be in 0~60 r/min and mixing power should be 5.5 kW when grain size met its requirement. Over-load of speed reducer and electric motor were prevented, and rigid deformation of anchor was reduced. The product yield and quality were improved.