Using the time-dependent multilevel approach (TDMA) and combining the B-spline expansion technique and the model potenial method of atom, the properties of Rydberg sodium atom are investigated. The energy level structure of high excited states and population transfer of sodium atom in a microwave field are studied by numerical calculation, and quantum states are controlled and manipulated by microwave field. The results show that the method we used can be applied to the investigation of the properties of alkali-metal atoms in external field, that the population is very sensitive to the parameters of chirped rate and field amplitude, and that the population can be completely transferred into the target state and trapped there by changing the chirped rate and field amplitude.