Faced with different sea conditions, the shipboard coherent S-band wave radar digital receiver needs to support 6 different baseband sampling rates and bandwidths. In order to meet the multi-functional integration of radar modules and the radar system's requirements for multi-rate and multi-bandwidth, an FPGA-based multi-bandwidth digital down-conversion（DDC）solution is proposed, which can dynamically load decimation multiples and filter coefficients according to the receiver operating parameters and suitable for different sea condition. A 16-stage pipeline numerically controlled oscillator（NCO）module is realized through the coordinate rotation digital computer（CORDIC）; the distributed algorithm（DA）is improved to realize a fully parallel 128-order FIR filter, and the DA channel is time-division multiplexed; the asynchronous FIFO module guarantees that data is transmitted without distortion across clock domains, and the data is appropriately truncated to save hardware resources. The final result shows that the baseband I/Q signal frequency is accurate, and the signal-to-noise ratio is as high as 60 dB, which meets the design goals and radar requirements. This structure can save a lot of multipliers and RAM resources. After the DA structure is time-division multiplexed, it can save an additional 42.8% Look-up table resource. The designed DDC is reasonable and feasible, which can effectively switch the decimation factor and filter coefficient according to different sea conditions, and provide a stable and effective data source for the subsequent baseband signal processor. This has important reference significance for the multi-functional integration of radar modules.