Problem statement. In accordance with the code of rules (SP 101.13330.2012), the use of fish protection devices with a filter screen and a flushing system applies to water intake facilities in various placement conditions, in this regard, it is relevant to expand the possibilities of using filter cassettes at shallow water depths in the water source. Results. A methodology has been developed to substantiate the filler parameters of a horizontal filter cassette in the form of plastic balls and their distribution along its length, as well as a preliminary assessment of the uniformity of flow rates over the area of the filter cassette. Conclusions. To improve the operational characteristics of the filter fish-protective device in conditions of shallow water sampling depths, instead of the basic design with a vertical cassette layout, a filter cassette with a horizontally positioned water intake surface and a self-washing effect is recommended, in which a filler made of plastic balls (with positive buoyancy) is used and there is free space for their vertical movement. On the basis of hydraulic calculation, a method for preliminary estimation of filtration rates in such a cassette has been developed. To ensure a uniform distribution of the filtration rates of the water flow along the length of the cassette, it is recommended to use balls of different diameters. Evaluation of the working conditions of the cassette allowed us to obtain a pattern of changes in the diameter of the balls along its length, so for a specific example, the calculated diameter of the balls varied from 8 mm, in the suction nozzle, to 40 mm at the end of the cassette. The technique can be used to calculate the parameters of horizontal filter cassettes of fish protection devices.