For studying the difference of theoretical value and the simulated value by MCNP of waken law fromγ-ray of narrow beam, Models based on MCNP are set up, Pb material and W material are chose to be the shielding material.Theoretical calculations are carried out based on the mathematical formula.Comparing the simulated value and the theoretical value, if conditions demanded by the mathematical formula are considered carefully when setting up the models, the consistency between simulated value by MCNP and theoretical value is very good, the difference is week, the request of engineering can be satisfied.%为了研究窄束γ射线减弱规律模拟值与理论值之间的差异；使用MCNP程序建立计算模型，选择了铅材和钨材作为屏蔽材料进行模拟计算；使用理论计算公式进行理论计算。比较模拟值和理论值发现，当计算模型的建立充分考虑了理论公式要求的条件时，MCNP模拟值与理论值的一致性很好，差异很小，可满足工程应用的要求。
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