The invention relates to a mechanical ultrasonic contrast agent injecting device. The mechanical ultrasonic contrast agent injecting device comprises a rectangular main shell, a main mounting vertical plate, a main lower positioning cylinder, a main upper positioning cylinder, a main lower sliding sleeve, a main upper sliding sleeve, a main lower supporting spring, a main upper supporting spring, a main pull rod, a main upper push column, a main upper piston push plate, a main left clamping piece, a main right clamping piece, a main syringe and a main lock. The mechanical ultrasonic contrast agent injecting device further comprises a rectangular auxiliary shell, an auxiliary mounting vertical plate, an auxiliary lower positioning cylinder, an auxiliary upper positioning cylinder, an auxiliary lower sliding sleeve, an auxiliary upper sliding sleeve, an auxiliary lower supporting spring, an auxiliary upper supporting spring, an auxiliary pull rod, an auxiliary upper push column, an auxiliary upper piston push plate, an auxiliary left clamping piece, an auxiliary right clamping piece, an auxiliary syringe and an auxiliary lock. The mechanical ultrasonic contrast agent injecting device provided by the invention has the benefits that an ultrasonic contrast agent in the main syringe is quickly injected into surrounding veins of an examinee, physiological saline in the auxiliary syringe is quickly injected into the surrounding veins of the examinee, and the diagnostic accuracy rate is greatly improved.