The invention relates to tread rubber for a high thermal diffusivity semisteel car tire, the tread rubber comprises the following effective raw materials: by weight, 50 parts of natural rubber, 20-30 parts of high vinyl polybutadiene rubber, 20-30 parts of solution polymerized styrene-butadiene rubber, 30-40 parts of polydimethylsiloxane, 1-5 parts of dithiophosphate, 4-7 parts of zinc oxide, 2-4 parts of stearic acid, 0.1-0.3 part of antiscorching agent CTP, 2-6 parts of microcrystalline wax, 5-8 parts of aromatic hydrocarbon, 1-2 parts of cobalt naphthenate and 2-5 parts of clay, by the interaction of each effective raw material, the themogenesis is low, and by adding of the polydimethylsiloxane as a reinforcing agent, the tread rubber strength is effectively increased, and the tread rubber is low in cost, low in carbon and environmentally friendly; in addition the tread rubber is also added with the cobalt naphthenate, so that tread and steel wire combination is more tightly, the strength of a tire made of the tread rubber is further enhanced, and the tire made of the tread rubber is good in thermal diffusivity, strong in ground catching performance and high in high speed performance.