The invention discloses a well lid anti-theft device. The well lid anti-theft device comprises a well lid outer frame and a well lid arranged at the inner side of the well lid outer frame, wherein a side frame which is in the shape of a hollow barrel is arranged on the middle part of the well lid outer frame; a ring-shaped baffle plate is arranged at the inner side of the side frame; two strip-shaped slots which are uniformly distributed are formed in the baffle plate; the end parts of the strip-shaped slots are equipped with supporting plates; fixed slots are formed in the supporting plates;cross sections of the fixed slots are in the shapes of L with opening facing leftward; the well lid is blocked to the inner side of the side frame; two cylinders which are uniformly distributed are arranged on the lower end surface of the well lid; the lower ends of the cylinders can be spliced into the corresponding strip-shaped slots; and the upper ends of the cylinders are rotatably connected onto the lower end surface of the well lid. According to the well lid anti-theft device disclosed by the invention, the well lid with a fixed rod is combined with the baffle plate with the supporting plates, so that the well lid can be blocked onto the baffle plate, and therefore, a good anti-floating function and a good anti-theft function can be achieved.