The Large High Altitude Air Shower Observatory (LHAASO)
will be constructed at Mt. Haizi in Sichuan Province, China. Among
several detector components of the LHAASO, the Water Cherenkov
Detector Array (WCDA) is of great importance for low-to-middle
energy gamma ray physics. The WCDA has an area of 78,000 $m^{2}$ in
total, which is sub-divided into 3120 cells by curtains, with a PMT
resided in each cell. Old simulation program for WCDA is based on
event stream which is very popular, but produce huge memory
consuming for large events, so this old method is probably not
suitable for treatment of events piling-up like WCDA simulation. A
new method of the simulation based on Hit stream is developed for
the LHAASO-WCDA, which not only can resolve the problem of memory
consuming up but also can do union-simulation for all the LHAASO
detector array.