To address the challenges of significant difficulty and insufficient accuracy in locating the centerline of small-diameter buried pipelines in urban underground short and medium-distance scenarios, this paper proposes an improved Rauch-Tung Striebel (RTS) filtering and smoothing algorithm aimed at enhancing positioning precision. The method is based on a combined navigation scheme using an inertial navigation system and an odometer. It initially employs an extended Kalman filter for forward data fusion to suppress error divergence. Subsequently, an innovative improved RTS backward smoothing algorithm is proposed. By introducing a smoothed "error state", it directly corrects the positioning information and sensor parameters. Through sensor parameter compensation and secondary filtering optimization, the positioning accuracy is improved. To validate the algorithm′s effectiveness, simulation experiments were conducted. The results indicate that this method can achieve pipeline centerline mapping accuracy on the order of 10-3, fully meeting the engineering application requirements for pipeline in-line inspection positioning. This provides more reliable data support for the safe operation and maintenance of pipelines.