Abstract: 3 effectiveness evaluation tests with coupon OFF method are carried out. The authors compare the test results with OFF potential measured by instantaneous synchronization OFF method with a potentiostat. The results show that for different types of pipe coating, coating condition and measuring point location, test results will appear different laws. A discussion is made on the real significance of the OFF potential measured by auxiliary coupon OFF method based on polarized coupon with electrostatic field theory. The conclusion is that for polarized coupon with particular area, the measured potentials are between the pipe/soil ON potential and natural potential of the coupon, the coupon with certain electrode size can not apply to the measurement under all conditions. The electrode area should be adjusted according to the type of pipe coatings, coating conditions and measuring point location, and calculation verification should be performed for basic law of OFF potential measured by the coupon OFF method through numerical simulation. The relevant conclusions are also applicable to the polarized probe OFF method.