In order to integrate the “sponge city” with the urban drainage network, synergy can alleviate urban guilt. The study takes Tianjin as an example to construct the vulnerability evaluation index system of urban drainage network from the three dimensions of “exposure-sensitivity-adaptive ability”. The punitive variable weight comprehensive evaluation model was used to conduct the “diagnosis” of the vulnerability of urban drainage networks. The GM (1, 1) model in MATLAB was used to predict the vulnerability index of urban drainage network. The results indicated that the vulnerability index of urban drainage network in Tianjin fluctuated between 0.442 and 0.652, and the overall trend was upward during the period of 2008-2017. According to the forecast results, the trend extend to 2018-2020, and the vulnerability is steadily in the “light” range. Through comparison between the results of the variable power evaluation and the constant power evaluation, the comprehensive evaluation model of the punitive variable can eliminate the bubble in the constant power and improve the objectivity of the evaluation result.