The invention discloses a safety device of controlling hydraulic self-locking of a vehicle door by a relative traveling speed. The safety device comprises a vehicle door stroke control spherical hinge, a vehicle door stroke hydraulic executing module, a speed measuring device module, a singlechip microcomputer device module, a vehicle door stroke control switch and a vehicle door stroke hydraulic reversing valve assembly. The vehicle door stroke control spherical hinge and the vehicle door stroke hydraulic executing module are integrally connected and are mounted below a vehicle door; the singlechip microcomputer device module compares relative traveling specific speeds of passing vehicles in a speed measuring covering region with a safety speed set by the singlechip microcomputer device module; in a dangerous situation, the vehicle door stroke hydraulic reversing valve assembly is in a self-locking state, the relative travelling speed controls the vehicle door to be self-locked hydraulically and the vehicle door cannot be opened; in an emergency state or a manually judge safe state as well as a safe condition, the vehicle door stroke hydraulic reversing valve assembly is in a connected state, and the vehicle door can be freely opened. The safety device disclosed by the invention is simple in structure, low in manufacturing cost and high in safety, and is favorably promoted in automobile industry.