A polyurethane used for orthopedics external fixing system in complex environment is obtained by reaction of mixture of polyether polyol with modified isocyanate in the weight ratio of 1:0.6-0.75. The modified isocyanate is obtained by reaction of polyphenylene polymethylene isocyanate with polyether glycol. The components and corresponding weight percent amounts of the mixture of polyether polyol are as follows: 60-65% polyether polyol; 0.3-1% catalyst; 0.1-2% foam stabilizer; 0.1-2% or 3-20% foaming agent; 25-40% filler and 0.5-2% functional auxiliary agent. The method of producing the polyurethane is also provided.