The time-dependent multilevel approach (TDMA) and the B-spline expansion technique are used to study the properties of Rydberg lithium atom. The energy level structures of high excited states n = 7075, l = 05 and population transfer of lithium atom in a microwave field are studied by numerical calculation. The results show that the coherent control of the population transfer in microwave field from the initial to the target states can be accomplished by optimizing the microwave field parameters. and that each state plays a crucial role in the transition process.