Microbial biomass phosphorus (MBP) is one of the most active forms of phosphorus in soil. Soil MBP plays a pivotal role in the transformation and biogeochemical cycling of soil phosphorus, which is considered to be an important source of available phosphorus for plants. Exploring size of the soil MBP pool is of important significance to fully understand the potential of phosphorus fixation in microorganisms, and grasp the capacity of soil phosphorus transformation and circulation. Three typical soil-crop systems in farmlands of the Northern China were selected to investigate size of soil MBP pool in agroecological system. A total of 362 surface soil samples (0-30 cm) were collected from the research area based on fixed-point sampling. The results showed that there were significant differences in MBP content among three typical soil-crop systems. The average contents of soil MBP in the topsoil (0-30 cm) layer under black soil-spring maize, fluvo-aquic soil-winter wheat/summer maize and grey desert soil-cotton systems were 17.36, 14.45 and 8.75 mg/kg, respectively. Sizes of soil MBP pool of black soil-spring maize system, fluvo-aquic soil-winter wheat/summer maize system and grey desert soil-cotton system were 83.60, 54.26 and 39.80 kg P/hm², respectively, in the topsoil (0-30 cm) layer. In the three typical soil-crop systems, the amount of phosphorus stored in the soil MBP pool was 1.10-2.73 times of the amount of phosphorus required by one season crop, which indicates soil MBP pool is a huge source of available phosphorus for plants in agroecological system. Further analysis showed the size of MBP pool was influenced by the physical-chemical properties of soil and climate factors. A linear mixed-effects model showed that soil pH, organic carbon, mean annual temperature and mean annual precipitation are the main factors affecting the size of soil MBP pool, which can explain 57% of the variation of soil MBP in three typical soil-crop systems. This is the first study that size of soil MBP pool in different soil-crop systems is quantified in the farmland in the Northern China, which is of great importance to further understanding the potential phosphorus supply capacity of soil MBP pool. Our results indicate that soil MBP pool is a potential available phosphorus pool and may serve as a scientific basis for reduction of phosphorus fertilizer application and regionalized nutrient management. In the present study, we emphasize that the importance of microorganisms as a potential available phosphorus nutrient pool, and suggest regulating soil MBP pool size by changing content of soil organic carbon so as to utilize its phosphorus supply capacity in the future.