【Objective】To reveal the water loggogenic type of the fifth palaeosol in Xi’an area, and identify the water content, moisture balance and possible vegetation types when the S₅ developed.【Method】Field investigation, electron microscope observation and energy spectrum analysis were used to research relevant problem.【Result】The fifth palaeosol was composed of three layers, which was the best ancient soil in the loess. Based on the field investigation of the fifth palaeosol in Xi’an area, celadon spots were first discovered in the argillic horizon of this palaeosol, proving the accumulated water in the middle and upper paleosol and reducing environment during the development of S₅. The red ferruginous adhesive film developed very well, which has two kinds of micro structure under the electron microscope, one was crystal structure composed of newborn clay minerals, the other was amorphous colloid structure. According to the energy spectrum analysis, the content of Fe₂O₃、Al₂O₃ and SiO₂ in the red ferruginous adhesive film and celadon spots was between 12.85%～17.15%、20.45%～24.84% and 47.19%～54.17%, with average of 15.28%、23.07% and 52.13%, separately. Resulting from the deposition of green iron film on the surface of the red adhesive film, the difference of Fe₂O₃content between the red ferruginous adhesive film and celadon spots was quite small. The migration depth of red ferruginous adhesive film is 1.8m under the bottom of argillic horizon, and that was 6.3m from the top of the palaeosol.【Conclusion】These celadon spots in the fifth palaeosol were formed under the condition of deep ground water level. The major factor was abundant precipitation for reducing layer appearance above deep ground water level. The distribution depth of gravity water is up to 6.3m when the S₅developed and the climate would have been favorable to sustain forest development for a long period. At that time moisture content of reducing layer reached saturation state and that in the upper part of the soil was about 42%, generally speaking, soil moisture was quite adequate at then. During the development of S₅, the volume of receipts of soil water was more than the volume of expenditure, and the value of soil moisture balance was positive. Meanwhile, there was still sufficient atmospheric precipitation infiltrating to groundwater after evaporation, transpiration and surface runoff loess.