LHAASO is a large hybrid EAS experiment located in at Haizi Mountain, Daocheng, Sichuan province. KM2A is one of the main subarrays of LHAASO mainly for $\gamma$-ray astronomy and cosmic ray physics at energy above 20 TeV. The whole KM2A array consists of 5195 electromagnetic detectors (EDs) and 1171 muon detectors (MDs). The detectors construction is underway and a part of KM2A array have been operated since February 2018. The whole array will be finished until year 2021. Detector simulation is the important foundation for estimating detector performance and data analysis. While it is a big challenge to construct the KM2A detector simulation software in the framework of Geant4 due to large number of detector units ($>$6000), huge coverage (1.36 km$^{2}$), large altitude difference (30m), and huge photons tracing. In this talk, the design of KM2A array simulation code G4KM2A basing on Geant4 is introduced. The process of G4KM2A is optimized in memory consuming to avoid memory overflow. Some simplifications are used to significantly speed up the executing of G4KM2A. A comparison between MC and the part KM2A array data are also presented.