In a multi-wire proportional chamber detector(MWPC), the anode and signal wires must maintain suitable tensions, which is very important for the detector's stable and perfect performance. As a result, wire tension control and measurement is essential in MWPC's construction. The thermal neutron detector of multi-functional reflectometer at China Spallation Neutron Source is designed using a high pressure $^{3}$He MWPC detector, and in the construction of the detector, we developed a wire tension measurement system. This system is accurate, portable and time-saving. With it, the wires' tension on a anode wire plane has been tested, the measurement results show that the wire tension control techniques used in detector manufacture is reliable. ; Comment: 6 pages,12 figures