Objective To observe the effects of different doses of Ganoderma lucidum polysaccharide (GLP) pretreatment on hippocampal neuron apoptosis in rats undergoing cardiopulmonary bypass (CPB). Methods One hundred clean grade male SD rats weighing 350−450 g were divided into five groups according to the random number table method (n=20): sham operation group (group S), CPB group (group C), GLP low dose group (group G1), GLP medium dose group (group G2) and GLP high dose group (group G3). Under anesthesia, rats in each group underwent endotracheal intubation and mechanical ventilation with a light transmission method and catheterization of the right internal jugular vein, left femoral artery, and right femoral artery and vein. The model of CPB with a beating heart was established in groups C, G1, G2 and G3. CPB was performed for 1 h. The model was not established in group S but was observed for 1 h. Group G1, group G2 and group G3 were given 12.5, 25.0 μg/g and 50.0 μg/g, respectively, before operation GLP solution was gaved for 7 d, group S and group C were treated with the same amount of normal saline for 7 d. The serum was taken immediately after CPB and 5 h after CPB. The rats were killed under anesthesia 5 h after CPB. The hippocampal tissue was born, and the serum S100 calcium‑binding protein β (S100β) was detected by enzyme‑linked immunosorbent assay (ELISA) and neuron‑specific enolase (NSE) brain injury marker concentration. The expressions of B‑cell lymphoma/leukemia 2 (Bcl‑2) and Bcl‑2 related X protein (Bax) in the hippocampus were detected by Western blot, and the Bcl‑2/Bax was calculated. The terminal deoxynucleotidyl transferase‑mediated dUTP‑biotin nick end labeling (TUNEL) method was used to calculate the apoptosis index of neurons in the hippocampus. Results Compared with group S, the serum concentration of S100β and NSE increased significantly at immediately after CPB and 5 h (P<0.05), the expression of Bcl‑2 was down‑regulated (P<0.05), the expression of Bax was up‑regulated (P<0.05), Bcl‑2/Bax decreased (P<0.05), and the apoptotic index of hippocampal neurons increased (P<0.05) in groups C, G1, G2 and G3. Compared with group C, the serum concentration of S100β and NSE decreased significantly at immediately after CPB and 5 h (P<0.05), the expression of Bcl‑2 was up‑regulated (P<0.05), the expression of Bax was down‑regulated (P<0.05), Bcl‑2/Bax increased (P<0.05), and the apoptotic index of hippocampal neurons decreased (P<0.05) in groups G1, G2 and G3. Compared with group G1, the serum concentration of S100β and NSE decreased significantly at immediately after CPB and 5 h (P<0.05), the expression of Bcl‑2 was up‑regulated (P<0.05), the expression of Bax was down‑regulated (P<0.05), Bcl‑2/Bax increased (P<0.05), and the apoptosis index of hippocampal neurons decreased (P<0.05) in groups G2 and G3. Compared with group G2, the serum concentration of S100β and NSE decreased significantly at immediately after CPB and 5 h (P<0.05), the expression of Bcl‑2 was up‑regulated (P<0.05), the expression of Bax was down‑regulated (P<0.05), Bcl‑2/Bax increased (P<0.05), and apoptosis index of hippocampal neurons decreased (P<0.05) in group G3. Conclusions GLP pretreatment can regulate the imbalance of Bcl‑2 and Bax protein in brain tissue in a dose‑dependent manner and inhibit the apoptosis of hippocampal neurons, reducing the brain injury of rats under CPB.