To attain superior fire safety epoxy resins (EP), aluminum diethylphosphonate (AlPi) and nickel alginate were incorporated to EP in different proportions. The synergistic flame retardant effects between AlPi and nickel alginate on fire safety and mechanical properties of EP were investigated in detail. EP/AlPi9.5‐Nickel Alginate0.5 acquired the UL‐94 V‐0 rating with the highest limiting oxygen index value (28.9%). Besides, the thermal decomposition behaviors of the samples were researched by thermogravimetric analysis, implying that EP/AlPi‐Nickel Alginate exhibited the better thermal stability and char‐forming ability. Compared with EP, the peak heat release rate and total heat release were declined by 58.3% and 12.8%. And the addition of nickel alginate reduced the release of smoke. In particular, the incorporation of AlPi and nickel alginate increased the impact strength, flexural strength and glass transition temperature of EP. In perspective, the synergistic effect of bio‐based nickel alginate and AlPi opens a practicable avenue in decreasing the fire risk and improving the mechanical properties of EP.