Abstract: According to the characteristics of high density and high viscosity of crude oil in Binchang block of Ordos basin, the causes of crude oil thickening were studied by analyzing the geochemical characteristics of crude oil. The results show that the high viscosity crude oil in Binchang block was formed by the combined action of primary and secondary factors: ① The crude oil in Binchang block was formed in reduction environment, and the R_O of Chang 7 oil shale of source rock was 0.5%~0.8%, and the maturity was low, which led to the high heavy component of crude oil. ② Faults and fractures are widely developed in Binchang block, and the reservoir is vulnerable to late transformation. The soluble hydrocarbons in crude oil are selectively dissolved due to fault communication with formation water, and the loss of light components in crude oil during oil migration leads to crude oil thickening.