The invention relates to a phosphorus-nitrogen-boron synergistic efficient flame retardant flexible curing agent and a synthesis method thereof. The curing agent comprises the following components: 5to 16.7 parts of polyhydric alcohol, 18.7 to 42.5 parts of a phosphorus-nitrogen reactive flame retardant, 3.7 to 14.3 parts of a boron reactive flame retardant, 28 to 39 parts of polyatomic acid, 0.03 to 0.25 part of a catalyst, and 8 to 32 parts of an end-capping reagent; the acid value of the curing agent is 50 to 200 mgKOH/g; the melt viscosity is 800 to 7000 mPa.s/180 DEG C; the softening point is 60 to 125 DEG C; the P content is 2.7 to 6.4 wt%%; the N content is 1.2 to 2.9 wt%; the B content is 2 to 8 wt%. The existence of phosphorus, nitrogen and boron flame retardant elements and flexible chain ensures that the curing agent has excellent flame retardant efficiency, flame retardant durability and flexibility.