To investigate the key factors of design for thermo-expandable sheets, we made thermo-expandable sheets from two types of glass mat with different fiber diameter and PA thermoplastic film. Different thermo-expandable sheets were prepared by changing the fiber diameter, resin content and density, and their thermal expansion rate and effect were studied. The results show that the thermal expansion effect of thermo-expandable sheets is codetermined by the elastic potential energy of fiber and the viscous resistance of melting resin. Under the same composition and density condition, the thermal expansion rate of thermo-expandable sheets increases with the increase of fiber diameter. There is a critical resin content for thermo-expandable sheets. When the resin content is below the critical value, the thermal expansion rate increases with the increases of resin content. When the resin content is above the critical value, the thermal expansion rate decreases with the increase of resin content. The critical resin content decreases with the increase of fiber diameter. When the single fiber diameter and resin content is unchanged, the higher the density of thermo-expandable sheets, the greater the thermal expansion rate.