Since the gap between the probe electrode and the substrate electrode of scanning electrochemical microscopy (SECM) is not easy to control and clear, based on the feedback model and the COMSOL numeric simulation, the effect of the moving distance of the step motor on the real distance is investigated. The effects of the oxidation film of aluminum alloy on the approaching curve and the test error of the distance on the kinetic parameters of electrochemical-chemical reaction are also investigated. The results show that the positive and negative feedback effect is related to the gap between the probe and the substrate electrode. The smaller the gap, the stronger the feedback effect, while the feedback effect is also impacted by the regeneration kinetics on the substrate electrode. The ratio of the step motor-controlled distance to the approaching and withdrawing distance from the substrate electrode to the actual moving distance of the tip is 0.843 and 0.568, respectively, which indicates when the stepper motor movement is labeled 1 μm, the actually distances are only 0.843 and 0.568 μm. The state of the oxide film on the surface of aluminum alloy affects the approaching curve from partial positive feedback to pure negative feedback, and the distance error of stepper motor movement leads to the error of chemical homogeneous reaction rate constant in EC reaction as high as 60%. Precise gap control is the fundamental for scanning electrochemical microscopy experiments.