To detect polychlorinated biphenyls (PCBs), a prototype of a porous ZnO sensor based on the surface photovoltage (SPV) mechanism working under visible light illumination at room temperature has been presented. The SPV of the porous ZnO sensor can be remarkably reduced under visible light illumination after PCB adsorption, and the reduction of amplitude is proportional to the population of adsorbed PCB molecules. We propose that the reduction of SPV response is due to trapping of the electrons in the surface states by the adsorbed PCBs. The lower detection limits of this new prototype sensor reach at least 2.2 mu mol/L for PCB29 and 1.1 mu mol/L for PCB101, respectively. So, it demonstrates great potential for practical application in trace detection of PCBs.