Applying deep banding fertilizer is considered an effective strategy to improve grain yield and optimize fertilization management. However, the effect of P fertilizer application depth on P availability in the soil and the key mechanisms enabling the banding of P application to increase yield and efficiency under different soil layers, particularly in the field, remain unclear. We designed a field experiment using winter wheat (Triticum aestivum L.) to explore the effects of different P fertilizer placement depths of 8 cm (DP8), 16 cm (DP16), and 24 cm (DP24) on P availability and root spatial distribution. When compared with No P (no P fertilizer), the available P content of DP8, DP16, and DP24 in the corresponding soil layer increased by 30.0%, 82.4%, and 150.7%, respectively, in the two seasons. This increase in P availability caused a significant increase in the inorganic P (Pi) fractions of dicalcium phosphorus (Ca2-P), octacalcium phosphorus (Ca8-P), aluminum phosphorus (Al-P), and iron phosphorus (Fe-P), particularly in the 8-16 cm soil layer. Based on the increase in available P, especially the accumulation of Pi, in different soil layers, the root length density, root surface area density, and root activity were improved. The deeper P application treatments (DP16 and DP24) obtained 9.6%-25.2% and 80.1%-209.5% higher P uptake and phosphorus-use efficiency, respectively, compared with those of the traditional fertilization depth treatment (DP8). The grain yield increased by 17.0%-19.7% and 5.2%-8.1% in the DP16 and DP24 treatments, respectively, compared to that in the DP8 treatment. Additionally, the deep banding P application improved the aboveground biomass and harvest index, particularly in DP16, with an increase of 11.8%-16.3% and 2.9%-4.6% compared with that after DP8 treatment, respectively. Consequently, deep banding P application improved soil P availability and plant growth in the sub-humid area, especially when provided at a depth of 16 cm. Similarly, understanding this mechanism and the interaction between different deep banding P application depths with crops provides a practical reference for optimizing fertilizer management.