The object dimension measurement in small scenes has problems of low resolution, weak texture and lack of reference objects. To address these issues, a method using the 5-dof electromagnetic tracked endoscope is proposed. Firstly, the positioning principle of the endoscope based on the 5-dof sensor is analyzed. The displacement of endoscope along the main optical axis can be obtained. Then, the navigation acquisition method of the key information of the image target is analyzed. The contour information of the image target is obtained through the semantic segmentation network. Then, the coincidence degree with the main optical axis of the endoscope is determined, and the image target length information and the corresponding pose information of the key frame meeting the coincidence conditions are recorded. Finally, based on the pinhole camera imaging model, the target size measurement method is established by combining the target imaging scale relationship with the displacement of the endoscope along the main optical axis. Experimental results show that the measurement error of the method is controlled within 10% . The average measurement error for targets with a length of 1~ 9 mm is 0. 33 mm. It can meet the requirements of stable and reliable target size measurement, high precision, time-saving and laborsaving in the monocular endoscopy. Keywords:computer vision; object d