Based on the characteristic of the reservoir and the nature of the viscous crude oil of Pushen 18 oil region in Zhongyuan Oilfiled, the laboratory experiment on the rheological property and the law of the viscosity break by mixing the heavy oil with the thin oil were taken through. Under the constant temperature, the PS18-1 Ultra heavy oil was mixed with four different types of thin oil with different proportion, and the model parameters were obtained by fitting the experimental value. The result revealed that, under the same conditions, the best viscosity break efficiency was achieved when mixing the PS18-1 heavy oil with the Wen Yilian thin oil. Moreover, the bigger volume the thin oil mixed, the lower the viscosity turned into. In addition, the higher the wellhead temperature, the smaller amount of the thin oil needed. The heavy oil and the thin oil could not be mixed with the volume ration of 1∶1.5 without the viscosity reducer. The PS18-1 could be totally mixed with wenyi thin oil under 130℃ at volume ration of 1∶1.8, however, few cakes would be flushed out when the temperature descended. At the wellhead temperature of 40℃, the mixture oil viscosity was 249 mPa·s by mixing the heavy oil with the Wen Yilian thin oil at volume ratio of 1∶2, which could meet the production requirements. Besides, at the wellhead temperature of 60℃, the mixture oil viscosity was 356 mPa·s at the volume ratio of 1:1.8, which also could meet the production requirements. In addition, the model calculation fit well with the experiment value and had high accuracy degree.