To investigate the synergistic flame retardant effect of various elements, three flame retardants (SMAE, SDPS, SDPB) with similar chemical structure and containing different flame retardant elements were designed, synthesized, and applied to TPU. The test results showed that SMAE had a phosphorus-nitrogen synergistic effect and primarily exerted its flame retardant effect in the gaseous phase. The limiting oxygen index value of the TPU/4SMAE reached 32 % and passed UL 94 V-0 rating test. SDPB was obtained by introducing the boron, which further enhanced the flame retardant effect in the gaseous phase. The thermogravimetric combined with Fourier transform infrared spectroscopy results showed that TPU/4SDPB had the lowest intensity of gaseous phase products. With only 2 wt% SDPB, TPU can pass UL 94 V-0 test. Therefore, it also demonstrated that there was a good synergy between boron and phosphorus and nitrogen. SDPS was obtained by introducing the silicon, which can exert the flame retardant effect in the condensed phase. Scanning electron microscopy results showed that char layer of TPU/SDPS was the densest and silicon was retained as Si-O and Si-C in the char layer. However, the gaseous phase flame retardant effect of SDPS was restrained, and TPU/4SDPS only passes the V-2 rating. Hence, this work provided the direction of Schiff base derivatives flame retardants structure design in TPU. Display omitted •Three Schiff base derivatives (SMAE, SDPS, SDPB) were designed and synthesized.•4 wt% SMAE or SDPB endows TPU with LOI values over 30 %, passing UL 94 V-0 rating.•SDPS containing P, N and Si elements has the best barrier and protective effect.•2 wt% SDPB containing P, N and B elements can make TPU pass UL 94 V-0 test.•TPU/SDPB had the lowest intensity of gas phase products.