The invention provides a highly-transparent optical polyester film and a preparation method thereof. The highly-transparent optical polyester film comprises a layer A, a layer B and a layer C which are integrally formed into a three-layer stratified structure, wherein the layer A is a surface layer; the layer B is a sandwich layer; the layer C is a bottom layer; the layer A comprises the following raw materials in percentage by weight: 85-95% of modified polyester for films, and 5-15% of nanometer polymethylpentene particles or nanometer styrene butadiene resin particles; the layer B comprises the following raw materials in percentage by weight: 95-99% of super bright polyester for films and 1-5% of a heat stabilizer; the layer C comprises the following raw materials in percentage by weight: 85-95% of modified polyester for films, and 5-15% of the nanometer polymethylpentene particles or the nanometer styrene butadiene resin particles. Non-transparent anti-bonding particles (an anti-bonding agent) which are commonly used in the prior art need not to be added into the raw materials of the high-transparent optical polyester film, so that the processability of the high-transparent optical polyester film is improved; meanwhile, fogging is avoided, and the light transmittance and the glossiness of the high-transparent optical polyester film are improved.