Abstract: Well JHW00421 is the first shale oil horizontal well with ultra long horizontal section in Jimusaer Oilfield, and the designed length of its horizontal section is 3 027 m. It faces a series of drilling difficulties, such as large drilling friction and torque, instable borehole, difficult hole trajectory control, difficult hole cleaning and difficult production casing running. In order to solve these technological difficulties, the following technological researches and field practices were carried out. First, the casing program was optimized and the Ø244.5 mm technical casing was run to target A for horizontal section only. Second, the rotatory steering tool was adopted to control the trajectory of horizontal section accurately and reduce the borehole curvature. Third, the oil-based drilling fluid with oil/water ratio of 85∶15 was applied to reduce the wellbore instability risks of long horizontal section and the friction coefficient between drill string and wellbore. Fourth, based on the “transmission band” theory of horizontal well cleaning, wellbore cleaning technological scheme was formulated in detail to ensure the wellbore “clean”. Fifth, the model for calculating the friction of casing string was optimized and the friction coefficient was selected reasonably. In addition, the running capacity of casing string was predicted accurately and the casing running scheme was optimized. Sixth, the multi-centralizer drifting bottom hole assembly (BHA) was adopted and the type and setting position of casing centralizer were optimized to reduce the resistance to casing running. Seventh, the oil-based flushing spacer fluid was adopted and the cementing parameters were optimized to improve the cementing quality of horizontal section. The total depth of this well is 5 830 m and the actual length of the horizontal section is 3 100 m. According to the logging interpretation, the cementing quality of the horizontal section is of high quality. The research results provide reference and guidance for the design and construction of similar horizontal wells with ultra long horizontal section in the future.