The invention discloses a manhole cover anti-theft device. The manhole cover anti-theft device comprises a manhole-cover outer frame and a manhole cover arranged inside the manhole-cover outer frame. The middle part of the manhole-cover outer frame is provided with a hollow tubular-shaped border. An annular baffle is arranged inside the border. Two evenly-distributed strip-shaped grooves are formed in the baffle. The ends of the strip-shaped grooves are provided with support plates. Fixing grooves are formed in the support plates. The cross section of the fixing grooves is of right-angle shape. The manhole cover is connected with the inside of the border in a clamping mode. Two evenly-distributed cylinders are arranged at the lower end face of the manhole cover. The lower ends of the cylinders can be connected with the inside of the corresponding strip-shaped grooves in an inserting mode. The upper ends of the cylinders are connected with the lower end face of the manhole cover in a rotating mode. As the manhole cover anti-theft device is provided with the manhole cover with a fixing rod and the baffle with the support plates, the manhole cover can be connected with the baffle in a clamping mode and sound anti-floating and anti-theft functions can be achieved.