Aiming at the acid feature of the antibiotic medicine, based on the neutralization curve and the original battery reaction principle, when the pH value is 3.0 ~ 6.5, and by using 8 jars of standard solution as indicator, the linear relation between the pH value and the electric current can be drawn by using a simple circuit. According to this relation, the acidity strength of the wastewater under different electric current can be known. The disadvantages that on-line pH detectors are expensive and hard to be popularized in workshops can thus be overcome.