To investigate the effect of phosphorus fertilization on the regulation of photosynthetic and fruiting characteristics of winter wheat under different water treatments, and to clarify the mechanism of phosphorus application to regulate the photosynthetic productivity and promote the development of spike floret development into grains, three water treatments (severe drought treatment, W0; moderate drought treatment, W1; and normal water treatment, W2) and two phosphorus fertilizer treatments (no phosphorus fertilizer treatment, P0; and phosphorus fertilizer treatment, P1) were set up in 2020-2022 for pot experiment. Using the massive-spike variety Zhoumai 16 (V1) and the multi-spike variety Yumai 49-198 (V2) as experimental materials, we analyzed the effects of phosphorus on the photosynthetic and fruiting characteristics of winter wheat of the two types of varieties under different water treatments by determining the number of fertile florets and the number of grains in the different spikes (the basal, the central, and the apical parts of the spikes), as well as the net photosynthetic rate, chlorophyll content, chlorophyll fluorescence parameter, and sucrose content of leaves. The results showed that the phosphorus fertilization could effectively increase the number of fertile florets at anthesis (W10 stage) in winter wheat, especially the most significant effect on the number of fertile florets at the basal spike position (13.74%-27.01%), followed by the top (9.57%-20.19%), and then the middle (6.97%-14.01%). For the number of grains at maturity, the phosphorus fertilizer application increased the number of spikelets per spike as well as the number of grains in each spikelet under drought stress, and then significantly increased the number of grains. In addition, the phosphorus fertilization was effective in increasing the net photosynthetic rate, chlorophyll content, photosystem II maximum photosynthetic efficiency (F_v/F_m) and actual photochemical efficiency (Φ_PSII) of leaves of both types of varieties, and the effects on sucrose content varied according to water treatments, with phosphorus fertilization decreasing the sucrose content under drought treatments, and increasing the sucrose content under normal water treatments. Pathway analyses of the photosynthetic indexes of the two types of varieties with the number of fertile florets and the number of grains per spike respectively revealed that five photosynthetic indexes were positively correlated with the number of fertile florets and the number of grains per spike, among which the direct pathway coefficients of Pn and sucrose with the number of fertile florets were 1.001 and 0.435, and those with the number of grains were 0.996 and 0.626, which were much higher than those of other indexes. Finally, phosphorus application significantly increased yield by increasing spike number, grain number per spike and 1000-grain weight, among which grain number per spike reached significant level under different water treatments, spike number was significant under W1 and W2 treatments, and 1000-grain weight improvement effect was not significant. In summary, the application of phosphorus fertilizer under different water treatments could improve the photosynthetic productivity of winter wheat by regulating its photosynthetic performance to maintain normal physiological and metabolic functions, thus reducing the degradation and abortion of fertile florets and promoting the spike floret develop into grains, ultimately alleviating the damage caused by drought stress for the purpose of achieving high and stable yields. The results provide a theoretical basis and technical support for alleviating the adverse effects of drought stress on wheat growth through the application of phosphorus fertilizer.