With 2-year pot-grown grafted Citrus cv. Olinda as test material, this paper studied the effects of Apatococcus lobatus parasitization on its leaf photosynthesis characteristics. No significant differences were observed between slightly affected and control leaves in their total chlorophyll (Chl a + b) and carotenoid (Car) contents, net photosynthetic rate (P(n)), intercellular CO2 concentration (C(i)), primary maximum photochemical efficiency of PS II (F(v)/F(m)), quantum efficiency of noncyclic electron transport of PS II (phiPS II), and photochemical quenching (qP), but for medium and severely affected leaves, their Chl a + b, Car, P(n), F(v)/F(m), phiPS II and qP decreased by 23.85%, 26.49%, 43.3%, 4.5%, 35.1% and 22.5%, and 37.61%, 44.04%, 64.5%, 8.6%, 63.6% and 40.1%, respectively, while C(i) increased, with significant differences to the control. It could be concluded that the massive parasitization of A. lobatus caused a stress to orange plant, and non-stomatal limitation was the dominating factor of P(n) reduction.