Provided is a detection method of lubricating oil film thickness distribution of a cylindrical roller bearing. A probe selection criteria is determined, ultrasonic sensor probes are respectively installed on two end portions and the middle portion of a roller of the cylindrical roller bearing, water bath coupling conditions of each probe are guaranteed, simultaneously the bearing is fully lubricated, and reflected signal amplitude A0i of each ultrasonic sensor probe at the time is synchronously recorded in an idle state of the bearing; the bearing is stably operated after loading, reflected signal amplitude A1i of each ultrasonic sensor probe at the time is synchronously recorded, and ultrasonic signal reflectivity Ri= A1i/ A0i at the corresponding position is calculated; according to the ultrasonic signal reflectivity and an interface contact rigidity principle, corresponding reflectivity Ri is converted into oil film rigidity at the corresponding position, and then oil film thickness at the corresponding position is obtained according to the oil film rigidity and a film thickness principle; and a cartesian coordinate system is built, and the lubricating oil film thickness distribution of a line contact universe scope is drawn. The detection method can be applied to on-line monitoring of practical working conditions and is simple and accurate.