Straightness is the main shape and position errors of the large longitudinal welding pipe, which is the direct impact on the installation of transportation pipeline. Over-bend straightening must be carried out, while before straightening the deflection along pipe axis must be detected firstly. How to measure the deflection of pipe quickly and accurately is an urgent need. Some researches on the key technology of deflection detecting system for large longitudinal welding pipe were conducted. The design of the measurement system was completed, the deflection detecting system modules based on virtual instrument was developed and the physical simulating experiments of deflection curve were made. As a result the relative error of the measurement is 1.7%, which verifies the system has higher feasibility and reliability and can meet the actual needs.