Based on the water budget method and a river-network numerical model, the drainage effects of the rainfall-tide-combinations were analyzed through the scenario of a certain tidal polder in Shanghai. Factors involving the drainage criteria, such as the value of rainstorm duration, the determination of drainage degree, the possibility of starting regulation level and pre-reducing water level, the short and long-term implementation of river regulation, the land usage transition and the controlling of the water surface ratio, were analyzed. The results indicate that in the study area of 100 km², the river hydraulic factors and the scales of the drainage facilities calculated by both the water budge method and the numerical model fulfill the engineering accuracy. For a certain drainage scale, the peak water levels and their durations vary under the different rainfall-tide-combinations. The peak water level is relatively large under the “Matsa” case, yet the duration of the high-water-level stage is longer and the water level is higher in the later stage under the “63-year” case. The technical standards of drainage in a plain tidal polder are related to the design rainstorm recurrence interval, the design rainstorm duration, the drainage time, the degree of the drainage, the design tidal level and tidal process. In addition, the rainfall-tide-combinations can highly affect the scale of drainage ability.