Peatlands occupy about 4% of Poland’s area and perform important environmental and economic functions, including agricultural ones. In the last century, the majority of fen peatlands were drained for agricultural use. Drainage reduced the water retention of peat and initiated changes in their physical and water properties. In order to determine the rate and extent of changes in these properties, in the following years 1987, 2007 and 2018, studies were carried out within the Supraśl Dolna fen peatland, NE Poland, in two prognostic soil-moisture complexes: moist (PSMC-B) and dry (PSMC-C). Studies have shown that in the PSMC-B complex, the water table depth decreased to a maximum of 100 cm below ground level, while on the PSMC-C complex, the ground water decreased even to 150 cm below ground level. In the PSMC-B complex, peat mineralisation and physical-water properties changed to a small extent during the research period (statistically insignificant). In the PSMC-C habitat, the subsidence of peatland surface was observed (11 cm), bulk density increased, whereas the total porosity and full water capacity decreased. Deterioration in physical and water properties caused gradual degradation of drained fen peatland soils and their evolution towards organo-mineral soils. In order to slow down or even stop degradation processes in shallow and medium deep peat-mursh soils, there is a need to stop and regulate the outflow of water using efficient drainage facilities, as well as to carry out technical restoration works.