Electrostatic phenomena widely exist in various olefin polymerization fluidized bed reactors, due to the continuous collision and friction of dielectric particles and the low-humidity reaction environment. The measurement of particle charge density is essential for monitoring and controlling the electrostatic level as well as probing the hydrodynamic characteristics in a fluidized bed. In this work, a non-intrusive online measurement system, employing ring-shaped sensing electrodes and a pressure transducer, is designed and implemented on a fluidized bed test rig. A modified model considering the influence of spatial sensitivity of the electrodes is proposed to predict the particle charge density in the fluidized bed, based on the measured induced electrostatic current and pressure drop signals. Experimental results have demonstrated that the induced electrostatic current and pressure drop signals show remarkable similarity, with the two PSDs both mainly distributed within 0.5-5 Hz, which indicates close relationship between the two signals. The predicted particle charge density increases with the superficial gas velocity and decreases with the injection content of liquid anti-static agent (IAA), showing the same variation tendencies as that measured through a Faraday cup. When the electrode width is 20 mm, the mean absolute relative errors for the direct method, area method and envelope method are 12.7%, 14.7% and 13.5%, respectively, indicating that the direct method is the most reliable signal processing approach proposed in this work. (C) 2017 Published by Elsevier B.V.