Synchrosqueezing algorithm and time window are introduced to present a new damage index to detect structural time-varying damage based on continuous wavelet transform. The effectiveness of the proposed index is validated via a numerical example of simply supported beam with abrupt stiffness reduction, linear stiffness reduction and multiple point time-varying damage. The results demonstrate that the presented damage index can detect structural time-varying damage accurately and the selection of time window length has little influence on the value of time-varying damage index.