It is analyzed that the sampling method, by which the result of bacterial test is accepted when the defect number equals zero in 1 mL water sample in which the number of bacteria is less than 1 mL^-1, through Poisson distribution.And it is proved that the probability of the error determination of it is higher.The multifarious sampling methods are put forward, which can decrease the probability of the error determination of inspect result of pure water containing bacterial number≥1 mL^-1 to less than 5%.The selections of samples and principle of judge are also discussed.