The invention provides a directional drilling trajectory fitting method and device based on the length of a pipeline. The method comprises the steps that S1, known data are acquired; S2, a preliminarytrajectory curve is drawn by utilizing the known data, wherein the preliminary trajectory curve comprises a plurality of smooth arc curves, and the length of the arc curves meets the requirement of apreset length; S3, the accurate length of the pipeline and parameters of a drill rod are obtained, and fitting correction is carried out on the preliminary trajectory curve by using the parameters ofthe drill rod to obtain a plurality of fitted trajectory curves; and S4, the trajectory curve meeting a preset condition in the fitting trajectory curves is determined to serve as a fitted trajectorycurve, and a fitted trajectory curve is drawn. The directional drilling trajectory can be fitted according to the know pipeline construction basic data such as soil entry point coordinates, soil outlet point coordinates, horizontal spacing, the length of the pipeline, the soil entry angle and the soil outlet angle, and the precision of fitting results is high.