A new microwave imaging reflectometry (MIR) system for KSTAR is being developed based on the experience gained via the TEXTOR proof-of-principle system [H. Park et al., Rev. Sci. Instrum. 74, 4239 (2003)] which aimed to measure the poloidal image of the electron density fluctuations essential for transport studies. The KSTAR system will adopt a multi-frequency probe beam source in the range of 90∼ 100GHz (X-mode case), which will enable the measurement of 2-D (radial and poloidal) fluctuations of the multiple cut-off layers, simultaneously. The optical system of the MIR system will be combined with the 2nd ECEI system (identical to the first ECEI system [GS Yun et al., Rev. Sci. Instrum. 81, 10D930 (2010)]) on KSTAR. The design of the launching and receiving optics of the MIR system will be constrained in order to maintain the performance of the ECEI system and thus it is necessary to consider sharing the zoom lens of the ECEI system. This stringent constraint is a challenge considering the tight wavefront matching requirement to obtain proper images for a wide range of cut-off layers within the focal depth. This paper discusses the details of the MIR system design that is compatible with the 2nd ECEI system on KSTAR.