An overflow blocking apparatus for a tank lorry comprises: a main body (10) having a first cylinder (11) and a second cylinder (12) formed therein, a first air supply passage (13) connected to a side of the first cylinder, and a first air discharge passage (15) formed at one side; a piston (20) having one end connected to a rod (21) and the other end elastically supported with a spring (22); a lower main body (30) connected to the main body and having a lot hole (31) therein and a second air supply passage (32) and a second air discharge passage (33) connected to the wall of the lot hole; a lot (40) capable of reciprocating in the lot hole; a tube (50) provided in the form of wrapping the outer side of the lower main body; and a float (60) provided to be lifted by buoyancy of oil.