Fusarium head blight (FHB) is an important wheat disease worldwide; however, its effects on the physiological parameters in plants with different levels of FHB resistance remain unclear. Here, we evaluated the effects of Fusarium graminearum infection on yield and the photosynthesis-associated parameters Pn, Gs, and Ci of wheat flag leaves and determined the influence of FHB resistance. The FHB-resistant wheat genotype L699 and its susceptible sister line L661 were point- and spray-inoculated. Photosynthesis-associated parameters were subsequently measured using a modulated photosynthesis system, and FHB intensity was evaluated. Compared with L661, FHB caused more significant reductions in the net photosynthetic rate and stomatal conductance of flag leaves in L699. However, FHB caused a larger reduction in the 1000-grain weight and total grain weight per spike in L661 compared with L699. Independence sample t test showed that FHB resistance was significantly higher in L699 compared with L661. We concluded that under the conditions of the present study, FHB had a significantly greater effect on net photosynthesis in the resistant line compared with the susceptible line; however, it had a greater impact on yield components in the susceptible line. These results provide a new insight into the physiological cost of host resistance to FHB.