Knowledge of the processes governing salt intrusion in estuaries is important, since it influences the eco-environment of estuaries as well as its water resource potential in many ways. Analytical models of salinity variation offer a simple and efficient method for studying salt intrusion in estuaries. In this paper, an unsteady analytical solution is presented to predict the spatio-temporal variation in salinity in convergent estuaries. It is derived from a one-dimensional advection-diffusion equation for salinity, adopting a constant mixing coefficient and a single-frequency tidal wave, which can directly reflect the influence of the tidal motion and the interaction between the tide and runoff. The deduced analytical solution is illustrated with an application to the Humen estuary of the Pearl River Delta (PRD) and proves to be an efficient and accurate approach for predicting the salt intrusion in convergent estuaries. The unsteady analytical solution is tested against observations from six study sites to validate its capability to predict intratidal variation in salt intrusion. The results show that the proposed unsteady analytical solution can be successfully used to reproduce the spatial distribution and temporal processes governing salinity dynamics in convergent, well-mixed estuaries. The proposed method provides a quick and convenient approach for deciding on water-fetching methods to make good use of water resources.