A new high waterproof and flame-retardant material was developed by using polylactic acid as a binder. Fireproof composite materials have excellent physical, mechanical, and flame-retardant properties. The water resistance property of fireproof composites increases as the polylactic acid doping ratio increases. Also, the mechanical performance test showed that the flexural strength of the composite materials reached 14.75 MPa a polylactic acid amount of 25 wt%. More so, the surface of the inorganic materials (magnesium hydroxide and aluminum hydroxide), were uniformly covered by the polylactic acid providing good interfacial compatibility between the inorganic materials and polylactic acid. The results of scanning electron microscopy showed that the fracture cross-section of the composites was smooth and dense. Thermogravimetric analysis and fire resistance testing further confirmed that the fireproof materials have good thermal stability. The as-prepared composite materials with improved physical and mechanical properties could be utilized to partitions in public buildings, non-load bearing walls, and a sandwich for fire doors.