To prepare high expansion foams, the crystal melting behavior of linear polypropylene (LPP) under different CO2 pressures was studied using an in-situ visualization method. It was found that with increasing temperature, the crystal boundaries were gradually melted. However, the melting window of the crystals was narrow. In this narrow window, we thought the high expansion LPP foams could be prepared by batch foaming. To verify this, foaming experiments were carried out. The results showed that at the temperature when crystals were nearly melted, the foam had an ultra-high expansion ratio of 45. This demonstrated that the semi-molten state was an important factor for preparing high expansion foams. The thermal insulation and compression performance of the foams were also studied. The lowest thermal conductivity was 37.2 mWm(-1) K-1. The compression tests showed that the collapse stress decreased with an increasing expansion ratio and increased with a decreasing cell size.