To further improve the efficiency of intumescent flame retardant (IFR) system consisting of ammonium polyphosphate and charring-foaming agent, MoO3-ZrO2 solid acid is used as an adjuvant. The influence of MoO3-ZrO2 on the thermal decomposition, reaction to small flame and fire behavior of polypropylene (PP)/IFR composite is investigated. The incorporation of 1 wt% solid acid results in the enhanced limiting oxygen index (LOI) and fire safety of PP/IFR composite, while decreasing its peak heat release rate from 431 to 370 kW/m(2). Flame retardant mechanism is proposed based on the analysis of thermal decomposition behavior and char residue. The yield and graphitization degree of char residue are improved by MoO3-ZrO2. The release of CO2 from thermal decomposition of IFR is accelerated by this solid acid. This work provides a new path to designing highly fire retardant polymer composites. (C) 2018 Elsevier B.V. All rights reserved.