The binomial distribution, the Poisson distribution and the normal distribution are several important distribution patterns in Probability & Statistics . The binomial distribution is a very important discontinuous variable distribution, its trial model is independent Bernoullii experiment, usually records random variable ～.In the binomial distribution ～，when is big, the computation load is big. The Poisson theorem, De Moivre- Laplace theorem have given the binomial distribution approximate calculation formula.
Through probability values’ calculation and the contrast of the binomial distribution, the Poisson distribution, the Normal distribution in this paper, obtains when is small (requests0.1 generally) and is not too big(requests 20 generally), the product5 ,that the Poisson distribution may use approximate well, but when is big enough, >5 and >5, we may use the normal distribution to carry on the approximate calculation to the binomial distribution.
Keywords：binomial distribution;Poisson distribution;nonmal distribution;approximate
