The M40 fiber reinforced bisphenol A dicyanate ester(BADCy)composites with soft buffer layer of oxazoline ring were prepared by coating epoxy E51 on the surface of M40 fiber. FTIR, SEM and mechanical investigation shows that epoxy E51 reacts with cyclotrimerization and forms oxazoline ring, which has high impact strength andflexural strength. The buffer oxazoline ring between the M40 fiber and BADCy matrix can relax the interface stress emerging from difference of CTE between M40 fiber and BADCy matrix, thus resulting in the improvement of interlaminar properties of M40/ BADCy pomposities. When the concentration of the treating solution is 5wt %, the interlaminarshearing strength (ILSS) and flexural strength of M40/ BADCy composite rise from 68. 9 MPa and 1080 MPa to 78. 1MPa and 1 110 MPa, respectively.