Massive MIMO is a key feature for improving spectral efficiency in 5G system. Since massive MIMO has been standardized in 3GPP Re1-13 (LTE-Advanced Pro), a next generation of massive MIMO standard is now available in Rel-15, a.k.a New Radio (NR), for 5G, which is aiming at further improving the average spectral efficiency with more antenna elements over the higher frequency band. In particular, beam-based air interface for above 6GHz band involves with various antenna configurations and feedback schemes, requiring a more complex testbed for system-level performance evaluation. In this paper, we examine the multi-antenna technologies in 3GPP NR specification, so as to develop its system-level model in 5G K-SimSys, which has been designed and implemented to provide an open platform and testbed for evaluating the system-level performance for 5G system. Its actual implementation is detailed along with the overall platform architecture for modular and flexible design. Then, its actual performance is demonstrated for one of the precoding matrices in the NR specification as an example.