Si-MAPP and Si-MMCA were prepared by modifying ammonium polyphosphate (APP) and melamine urate (MCA) with tetraethylorthosilicate (TEOS) and vinyltriethoxysilane (A-151). And a new intumescent flame retardant (IFR) was obtained by mixing Si-MAPP, Si-MMCA and dipentaerythritol (DPER) and used for LDPE flame retardant. The structure, morphology and mechanical properties of flame retardants and LDPE composites were characterized by FTIR, SEM, TG and universal material testing machine. The flame retardant properties of LDPE composites were investigated by LOI, UL-94 and cone calorimetry tests. The results show that Si-MAPP and Si-MMCA have excellent hydrophobic properties and good compatibility with LDPE after being modified by TEOS and A-151. And the problem of poor hydrophobicity of APP and MCA was resolved. The flame retardant properties of LDPE composites are greatly improved by adding IFR containing Si-MAPP/DPER/Si-MMCA. FTIR, SEM and TG results showed that APP and MCA were successfully modified by TEOS and A-151 coatings. The flame retardancy of LDPE composites was tested by universal material testing machine, limiting oxygen index, UL-94 vertical combustion test and cone calorimetry test. When 43.75 wt% Si-MAPP/DPER/Si-MMCA was used, the LOI of LDPE composites was 30.3% and reached V0 level. And the tensile strength reached 12.92 MPa, which was 3.79% higher than that of LDPE/IFR without Si-MMCA in the same proportion and 6.81% higher than that of 41.6 wt% LDPE/IFR without Si-MAPP. The smoke density tests show that the addition of Si-MAPP and Si-MMCA effectively improve the residual carbon content and thermal stability of LDPE composites, and significantly improve the smoke emission.