Abstract: Abstract：China's urban sludge production is huge. The harmless and resource-based disposal of sludge is imminent.Agricultural use is an effective way to dispose sludge after its harmless treatment. Calcareous soil is one of the soil typeswidely distributed in arid and semi-arid areas of China. The purpose of this study was to clarify the impact of sludge onnutrient status and fertility characteristics of calcareous soil by continuously adding sludge on calcareous soil and to explorethe critical value of phosphorus leaching from calcareous soil by adding sludge, so as to provide support to the rationalapplication of sludge in this kind of soil. The typical calcareous soil in the north of China was selected as the object of thisstudy, and the added amount of composted sludge was in accordance with the sludge application amount specified in thecontrol standard for pollutants in agricultural sludge (GB4284-2018). The five treatments of sludge addition amount weredesigned: 0 t/hm2(Control), 3.75 t/hm2(H1), 7.50 t/hm2(H2), 37.5 t/hm2(H3), 75.0 t/hm2(H4). The pot experiment of two years'continuous planting of winter wheat and summer maize was carried out to analyze the annual and seasonal changes of soilnutrients after adding different levels of composted sludge. The integrated fertility index was determined by Nemero indexmethod according to the relationship between soil available phosphorus Olsen-P and soluble phosphorus CaCl2-P. Therelationship between sludge addition and phosphorus leaching critical value was determined. The results showed that the pHvalues of calcareous soil decreased with the addition of sludge, but the content of soil organic matter (OM), total nitrogen(TN), available nitrogen (AN), total phosphorus (TP), available phosphorus (Olsen-P), available potassium (AK) and soilintegrated fertility index (IFI) increased with the increase of sludge addition. During the experiment, the content of soilnutrient in maize season was lower than that in wheat season when the sludge addition was the same in the same productionseason, and the nutrient content increased with the increase of sludge application years. There was a significant (P<0.05) orextremely significant (P<0.01) positive correlation between the nutrient content and the cumulative application amount ofsludge. When the amount of sludge added in each growing season was more than or equal to 37.5 t/ hm2, the soil nutrientcontents of treatments were significantly different from those of control treatments (P<0.05). The phosphorus leachingthresholds calculated by the above method was 28.57 mg/kg in calcareous soils. At this time, the amount of sludge applicationin the field was 61.39 t/hm2. The addition of sludge improved the nutrient and fertility status of calcareous soil, and attentionshould be paid to the risk of soil acidification and phosphorus leaching caused by excessive application of sludge.