By using B-spline basis set combined with model potential,energy levels and wave functions of rubidium atoms are obtained.Using a time-dependent multi-level approach,we study the dependence of the population of rubidium atoms excited from a low lying state to a target state on the parameters of a single or two broadband terahertz laser pulses.The population redistribution between the states n =23 and n =24 due to the interaction with terahertz pulses is analyzed in detail.Population oscillation in the initial and final states as a function of the time delay between two half cycle pulses is shown,and the oscillation period is the same as the Kepler period of the selected states.The calculation results agree well with the experiment and can be explained by the semiclassical picture.