The work aims to study the influence of fire-retardant treatment on the flame retardancy and physical properties of corrugated paperboard. Inorganic halogen-free composite flame retardant was prepared by blending guanidine phosphate, nano-silica, 4A molecular sieves which was modified by NH4Cl and Cu(NO3)2 and methyl cellulose. Flame-retardant corrugated paperboard was prepared by ultrasonic impregnation. According to the national standard, the main physical properties of corrugated paperboard were measured, and the flame-retardant effect of corrugated paperboard was tested by vertical combustion test and limiting oxygen index (LOI). Thermo gravimetric analysis (TG), field emission scanning (FE-SEM), X ray energy spectrometer (EDS) and Fourier transform infrared spectroscopy (FTIR) were used to characterize thermal stability, carbon residue rate, micro morphology, element composition content and change of chemical functional groups before and after flame-retardant treatment. The water absorption, edge compressive strength and burst strength of corrugated paperboard were increased by 33.3%, 53.7% and 14.5%, respectively. The peeling strength was decreased by 4.5%. The char length of flame-retardant corrugated paperboard was 20.3 mm, which reached technical performance index of flame-retardant board in GB/T 14656—2009, and the oxygen index reached 28% (47.4% higher than the control). TG analysis indicated that the carbon deposition rate of flame-retardant corrugated paperboard was increased by 111.09%. The results of FE-SEM, EDS and FTIR showed that there were micron-sized particles on the fiber surface of the flame-retardant corrugated paperboard. Other than C and O, the elements of these particles also included such components of flame retardants as N, P and Cu. The application of composite flame retardant can improve the flame retardancy and physical strength of corrugated paperboard. In addition, both the covering effect of flame retardant and the gas produced by the thermal decomposition during combustion play an important role in inhibiting the combustion with respect to the solidified phase flame resistance and the gas phase flame resistance.