This study aimed to improve the flame-retardant properties of laminated bamboo lumber (LBL) using phosphorus-nitrogen-boron flame retardants (FRs). The combination of a 7:3 ratio of monoammonium phosphate (MAP) and boric acid/borax compounds (SBX), and 74.32 kg/m3 of FRs (10.3% weight gain), exhibited enhanced fireproofing performance for LBL materials. A commercial flame retardant (guanylurea phosphate) (GUP) was systematically studied as a comparison. A cone calorimeter and a thermal analyzer were used to characterize the combustion behavior and thermal stability, respectively. The flame retardants morphology in bamboo cell cavities was investigated using scanning electron microscopy (SEM) and an energy dispersive X-ray analysis (EDXA). The results showed that at a heat flux of 50 kW/m2, the heat release rate and the total heat release of LBL samples treated with MAP-SBX flame retardants decreased more considerably than that of the untreated samples. The use of MAP-SBX not only promoted carbonization of LBL greatly but also indicated a good performance of smoke and combustion suppression as well as for the GUP. Flame retardants were confirmed to penetrate into the cell cavities of the bamboo using SEM and EDXA.