A photoelectrical device for detection of bacterial cell density includes a substrate, a driving electrode layer, an AC power source and a photoelectric conversion layer. The driving electrode layer is disposed on the substrate and includes a central electrode and a peripheral electrode pattern surrounding the central electrode. A fluid sample is disposed on the driving electrode layer. The AC power source is electrically connected to the driving electrode layer, and used to produce a non-uniform alternating electric field in the fluid sample on the driving electrode layer for driving the target bioparticles to gather up on the central electrode to form a particle cluster. The photoelectric conversion layer is used for receiving a light detecting beam after passing through the particle cluster and outputting an electric current based on the optical density of light detecting beam. The electric current changes as a concentration of the target bioparticles changes.