An analytical model is presented to investigate the vibration and stability of a pretwisted blade under nonconstant rotating speed. Two coupled bending displacements in flapwise and edgewise directions are considered. The time-dependent rotating speed leads to nonautonomous systems in which parametric resonance can occur. Six parametric instabilities, including primary and combination resonances, are predicted by using multiple scale method. These instability predictions are compared with those from numerical results of a more detail model. Among all instabilities, the combination resonance when perturbed frequency near twice of the fundamental frequency is found to be most critical and sensitive to system parameters.