In the context of vision measurement systems, ports represent a typical harsh measurement environment. The presence of docking ships, operational port cranes, and heavy-duty vehicles often subjects vision measurement equipment to frequent vibrations. These vibrations can cause relative movement between the lens groups, leading to changes in the principal distance. Current vision measurement equipment lacks the capability for rapid on-site principal distance error compensation in port settings, resulting in swift and severe degradation of measurement accuracy. This limitation significantly hinders the application of this advanced non-contact measurement technology in ports. To address this issue, this paper proposes a principal distance compensation method based on the symmetrical constraints of port crane structures. Utilizing the principle of approximate threshold scanning, this method achieves efficient and precise compensation of principal distance errors. Furthermore, the paper introduces a quantitative weighting method for the confidence level of constraint points, further enhancing the accuracy of the compensation values. The experiments demonstrate that the method proposed in this paper can significantly improve the accuracy performance of visual measurement systems under conditions of slight variations in focal length. It can be effectively applied in mechanical structure scenarios with geometric constraints.