In the utilization and transformation of lagoon, the measure to reduce the inlet width is often adopted in order to increase the inlet depth. Because of this the desired results have not been achieved. On the contrary, lagoon environment is getting worse. Therefore, the relations among tidal inlet velocity, cross-sectional area, inlet length, phase lag and repletion coefficient as well as the effect of drift sand along the coast on the inlet have to be analyzed. On the basis of self- experience and studies, the proposal of transforming problematic inlet, such as using the method of diversion dike under mean tidal level and double profile of inlet as well as inlet dividing, has been put forward for different types of lagoons.