The invention discloses a river silt cleaning device and a construction method. The river silt cleaning device comprises a cleaning mechanism, a gate mechanism and a silt storage and storage tank, thecleaning mechanism is formed by splicing cleaning units, each cleaning unit comprises a bottom plate and drainage devices, the drainage devices are symmetrically arranged on the top surface of the bottom plate, two shunting pipes are symmetrically arranged on the bottom surface of the bottom plate, the shunting pipes are in communication with the drainage device, the shunting pipes are fixedly connected to a confluence pipe, the confluence pipe is connected with a sewage discharging pipe through a connecting pipe, and the sewage discharging pipe is connected with a silt storage and storage tank through a flow guiding pipe, according to the construction method, comprises the steps of river channel bottom cleaning, silt storage tank construction, pipeline construction, installation of the cleaning mechanism, installation of the gate mechanism, and the silt cleaning device runs in a test mode are provided. According to the design of the cleaning mechanism, the silt at the bottom of the river channel can be regularly cleaned, so that the labor intensity of manual operation is greatly reduced, the steps of the construction method are simple, the silt cleaning requirements of the riverchannels with different widths can be met, the labor intensity of manual operation is reduced, and resources are saved.