The invention provides modified poly (propylene carbonate), which comprises a cross-linked poly (propylene carbonate) with polyphenyl polymethylene polyisocyanates as a cross-linking agent, as well as a polymer of hexanedioic acid and 1,2-propylene glycol ester, wherein the mass ratio of poly (propylene carbonate) to the polyphenyl polymethylene polyisocyanates is 100:(0.1-2), and the mass ratio of the cross-linked poly (propylene carbonate) to the polymer of hexanedioic acid and 1,2-propylene glycol ester is 100:(5-20). A preparation method of the modified poly (propylene carbonate) comprises the following steps: blending and extruding the poly (propylene carbonate) and the polyphenyl polymethylene polyisocyanates to obtain the cross-linked poly (propylene carbonate); and blending and extruding the cross-linked poly (propylene carbonate) and the polymer of hexanedioic acid and 1,2-propylene glycol ester to obtain the modified poly (propylene carbonate). The modified poly (propylene carbonate) provided by the invention is good in thermal stability, high in tensile elongation at break, good in transparency and capable of being biodegraded completely.