distributed polarization coupling in high birefringence fibres. Detailed theoretical analysis of the measurement accuracy of the testing system is presented. The main part is a Michelson interferometer with a scanning arm driven by a stepping motor. The minimum polarization coupling intensity detectable by this system is-77 dB. The maximum fibre length that can be tested under this design is more than 1 km, and the spatial resolution is better than 50mm. The influence of the opto-electro-mechanical devices on measurement accuracy has been analysed in order to improve the performance-to-cost ratio. This testing system can also be used in coherence division multiplexing optical fibre sensors.