Abstract: It’s a key question to decide the number of sampling unit in the sequential sampling associated with the dynamic information characterismic of units.In the paper,the conception of"The number( n min )of basic sampling unit"was first put forward,and its count equation was given.When using sequential sampling,The number of units( n )in sampling design must be bigger than or equal to n min ( n≥n min ).Both n min and n max decide the useful range of the sequential sampling linear eqution.