In this paper, an ultrasensitive photoelectrochemical sensor was developed for the detection of oxytetracycline, which was based on BaTiO3 sensitized Cu3Mo2O9 nano-composites with excellent visible-light photoelectrochemical activity. Cu3Mo2O9 nanocrystals (NCs) with popsicle-like morphologies were synthesized through hydrothermal method and showed a very high photoelectrochemical signal. To achieve larger photocurrent density, nanostructured BaTiO3 was then deposited on the surface of Cu3Mo2O9 photocathodes, which led to 3-fold enhancement of the photocurrent as compared to pristine Cu3Mo2O9 NCs. In order to obtain the specific response to oxytetracycline, oxytetracycline aptamer was fixed on the Au/BaTiO3/Cu3Mo2O9/FTO. Under visible light irradiation, the obtained aptamer/Au/BaTiO3/Cu3Mo2O9/FTO exhibited high sensitivity, good selectivity, outstanding reproducibility and low cost for detecting oxytetracycline. Furthermore, there was a linear relationship between detected signal and the oxytetracycline in the concentration range from 5 x 10(-10) to 10(-7) M, with a detection limit (3 S/N) of 4.6 x 10(-11) M under the optimal conditions. It is expected that this new photoelectric active material may have potential applications in biosensors, photocatalysis and other related fields.