In order to study the flexural behavior of external formwork with composite fireproof and self-insulation, six specimens were tested under loading. The failure modes, mechanical properties, flexural strength and the correlation between flexural strength and elastic modulus of specimens were analyzed. The test results show that the failure patterns of external formwork with composite fireproof and self-insulation are all manifested as cracks in the middle span mortar layer at the bottom, and the crack direction is perpendicular to the longitudinal axis of formwork and runs through the whole width. The load-displacement curves of the specimens were basically consistent, and the average values of the maximum load, flexural strength and elastic modulus are 846.34 N, 2.59 MPa and 5 125 MPa, respectively, which meets the requirements of the requirements of the corresponding specifications and have good flexural behavior. The flexural strength and the elastic modulus of the external formwork with composite fireproof and self-insulation are at the 0.01 level (R=0.972 5), showing a strong linear correlation.