Graphitic carbon nitride (g-C 3 N 4 ) is a promising nonmetallic photocatalyst. In this manuscript, B-doped 3D flower-like g-C 3 N 4 mesoporous nanospheres (BMNS) were successfully prepared by self-assembly method. The doping of B element promotes the internal growth of hollow flower-like g-C 3 N 4 without changing the surface roughness structure, resulting in a porous floc structure, which enhances the light absorption and light reflection ability, thereby improving the light utilization rate. In addition, B element provides lower band gap, which stimulates the carrier movement and increases the activity of photogenerated carriers. The photocatalytic mechanism and process of BMNS were investigated in depth by structural characterization and performance testing. BMNS-10 % shows good degradation for four different pollutants, among which the degradation effect on Rhodamine B (RhB) reaches 97 % in 30 min. The apparent rate constant of RhB degradation by BMNS-10 % is 0.125 min -1 , which is 46 times faster compared to bulk g-C 3 N 4 (BCN). And the photocatalyst also exhibits excellent H 2 O 2 production rate under visible light. Under λ > 420 nm, the H 2 O 2 yield of BMNS-10 % (779.9 μM) in 1 h is 15.9 times higher than that of BCN (48.98 μM). Finally, the photocatalytic mechanism is proposed from the results of free radical trapping experiments.