The upflow microbial fuel cell （MFC） system with membrane-free air-cathode was developed, and landfill leachate as fuel. The anode was made of stainless steel mesh filled with activated carbon， and carbon felt as the cathode. Electricity production performance of MFC with cylindrical stainless steel mesh anode filled with 0 (unfilled), 1mm, and 3mm cylindrical activated carbon particles was investigated. The electrochemical process of anode was observed by the cyclic voltammetry test. The mechanism of anode electrode reaction was analyzed. The results showed that the electricity generation performance of MFC with the anode filled with activated carbon was better than that of unfilled MFC. Maximum volumetric density of 4056.4mW/m3 and the apparent internal resistance of 494Ω were achieved from MFC with anode filled with 1mm particle. With the decrease of filled particles size (anode surface area increase), the electrochemical activity of microbes enriched on the surface of graphite rod increased. The electrode reaction process of anode with unfilled and filled with 1mm and 3mm particle were an irreversible process, a reversible process and a partially reversible process.