In case it is inconvenient to interrupt the power supply or there exists stray current the cathodic protection test probe can be applied to evaluate the efficiency of the cathodic protection system. The measurement accuracy of the test probe is largely determined by its structure which is studied extensively in this work including the porous tip’s porosity and aperture the coupons’ geometry and distance as well as the type of the built-in reference electrode and electrolyte. Both the indoor and the outdoor experiments are carried out showing that the designed cathodic protection test probe performs well in measuring the polarized potential and the native potential.