The angular momentum projected shell model(PSM) is applied to the nucleus 84Rb. The results of theoretical calculations about the positiveparity yrast band with configuration πg9/2νg9/2 and the negativeparity yrast band with configuration π(p3/2,f5/2)νg9/2 are comparedwith experimental data;especially,the mechanism of signature inversion in the positiveparity yrast band is discussed.The interpretation within the PSM shows that this signature pattern is a signal of a substantial quadrupole shape change with increasing spin,where the nucleus evolves from a prolate shape at low spins through a triaxial shape to an oblate shape at high spins.In addition,the nuclear shape for these two bands are also specified.