The Fengcheng Formation in Mahu sag and the Lower Lucaogou Formation in which crude oil is accumulated in the oil layers of closely interbedded source and reservoir combination with self-generated and self-contained model, are considered as case studies to discover the transformation law of crude oil physical properties with thermal evolution degree of source rock.The transition law of crude oil physical properties with increased thermal evolution degree is determined based on (i) the analysis of physical properties, organic geochemistry and enrichment characteristics of crude oil; (ii) the relationship between physical properties of crude oil and thermal evolution degree of source rock; and (iii) comprehensively considering thermal simulation experiment results of source rock and domestic and foreign relevant literature materials. The density and viscosity of crude oil from a source rock has the characteristic of first increasing and then decreasing with increasing thermal maturity, with the highest value at the oil peak depth. This understanding is not only a further clarification and revision of the existing understanding of the evolution law of crude oil physical properties with thermal evolution in petroleum geology, but also a very important and realistic guidance for the selection of sweet layer and sweet spot of shale oil and the in-situ transformation of shale oil.