C125 ultra high performance grouting materials for offshore wind power were prepared by using azo compound plastic expansion agent (PEA1) and nitroso compound plastic expansion agent (PEA2), respectively. The effects of the type and content of plastic expansion agent on the fluidity, expansion rate, compressive strength, electric flux and pore structure distribution of C125 ultra high performance grouting materials for offshore wind power were studied. The results show that with the increase of plastic expansion agent content, the fluidity of grouting materials increases first and then decreases, the compressive strength decreases gradually, and the vertical expansion rate and electric flux increase gradually. Compared with PEA2, PEA1 has greater influence on the fluidity, compressive strength, vertical expansion rate and electric flux of grouting materials at the same content. The total void fraction, average pore diameter and large pore proportion of grouting materials with PEA1 increase more obviously than that of grouting materials with PEA2 at the same content.