The Water Cherenkov Detector Array (WCDA) is one of the major components of the Large High Altitude Air Shower Observatory (LHAASO). The whole WCDA is subdivided into 3120 detector cells, each has 5x5 m2 area with an 8 inch PMT residing in the water at the effective depth of 4 m. In order to extend the dynamic range of the detector to the energy more than 100 TeV for the cosmic ray spectrum measurement, 900 cells of the WCDA array are put another 1 inch PMT at side of the big PMT. A fast simulation code based on Geant4 is developed for purpose of studying the performance of the extension system. In this paper, the fast simulation method is introduced, and CORSIKA and Geant4 simulation results, such as the shower core and the lateral distribution of the shower signals for high energy cosmic rays is presented.