Monte-Carlo method was applied to calculate the annihilation probabilities of positrons from 22Na in Mylar and Ni films with different thickness. Results of substrate effects of Mylar films with different thickness for sample of Si, Fe, PE and PP were obtained. The results showed that the substrate effects for samples PE and PP were stronger than those for Si and Fe. Therefore it is feasible for using Monte-Carlo method to calculate substrate effect.%用Monte-Carlo方法模拟计算了22Na源的正电子在不同厚度Mylar膜、Ni膜衬底中的湮没几率，得出Si、Fe、PE、PP 4种样品在不同Mylar膜厚度下的衬底效应。结果显示，对PE、PP样品衬底效应较Si、Fe等样品强。表明用Monte-Carlo方法计算衬底效应是可行的。
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