Since the first pilot 75 t/h water cooled separator CFB boiler was in operation in 1996, a hot test program was been conducted by Tsinghua University and several boiler works, so as to promote the scaling of CFB boiler. The present paper introduces the test program and the primary results, such as the separator efficiency, material balance, solid bulk density, heat transfer along the height of the furnace, et al. Besides the traditional test technology, some new developed apparatus was used in the hot test. According to the experience obtained from the boiler, the improvement measure was taken for the second and third boiler, which were in commercial operation in 1998. The first operation experience of the second and third boiler proves the effect of the improvement. Considering the experience and the test program, the scaling up design of water cooled separator was done. The present paper also introduces the design consideration and feature of the 130 t/h and 220 t/h CFB boiler.