A device for injection of a hydrocarbon feedstock into a catalytic cracking reactor comprising, from upstream to downstream in the direction of flow of the feedstock, an intake and mixing system (5) for the liquid hydrocarbon feedstock and steam, a venturi tube (6) whose entrance cone (10) is joined to said intake and mixing system (5), and a protective cap (7) that is integral with the discharge cone (14) of the venturi tube and is provided with an orifice (16) for injection of the feedstock into the reactor. The dimensions of the venturi tube (6) are such that the velocity of the mixture of liquid feedstock and steam attains therein sonic conditions at the level of the throat (13), and the rectilinear throat located between the discharge cone (14) and the entrance cone (10) is connected to such cones continuously by a curved section without making an angle exceeding about 5 degrees and about 15 degrees, respectively, with the axis of the venturi tube.