To study the frost resistance mechanism of multi-scale polypropylene fiber reinforced concrete(MPFC), crude polypropylene fibers in one size and fine polypropylene fibers in two sizes were selected to single-dope and mix-dope with concrete. The mass loss rate, the relative dynamic elastic modulus variation, and the tensile and compressive strength of the polypropylene fiber reinforced concrete before and after freeze-thaw cycle were analyzed based on the fast freeze-thaw cycle test and compression and splitting test. Mercury intrusion porosimetry(MIP) was carried out to study the relationship between frost durability degradation of cement concrete and pore structure. Besides, the influence of pore structure on the anti-frost performance is analyzed. The research indicates that adding polypropylene fiber into plain concrete can improve both the microstructure and the frost resistance of concrete. Under the condition of the same total fiber addition, crude polypropylene fiber and multi-scale polypropylene fiber can bring great improvement on frost resistance of concrete, and the effect of multi-scale polypropylene fiber on frost resistance of concrete is the best. Compared with the common concrete after freeze-thaw cycle, the compression and tensile strength loss of single-dope of fine fiber are reduceded by 9.95% to 11.94% and 4.29% to 7.62% respectively while the strength loss of single-dope crude fiber is reduced by 27.36% and 16.67% respectively, and that of MPFC decreases by 46.77% to 53.23% and 41.90% to 50% respectively.