By assuming that (1)the lateral structure of an air shower follows the NKG function and (2)the Poisson distribution can be used to select real detective particle density for each particle density detector, in which the mean value is determined by NKG function (i.e.(1)),and making use of Monte-Carlo method, the effective area (i.e.acceptance) for the air shower array of Mt. Liang Wang Cosmic ray observation station is simulated numerically. The calculated response function from simulation values of effective area agrees well with the observed results of the EAS array experiment, and it is good to use simulation values of effective area to analyse experimental data of the EAS array. This fact confirms that the simple method of calculating effective area is proper and appicable.