Provided is a method for manufacturing a horizontal concrete shearing key of a tunnel immersed tube joint. When sleeper beams are prefabricated, axis reinforcement and connectors are buried between steel seal door locking piles, and vertical reinforcement and connectors are buried in the front ends; after butt joint of a front immersed tube section and a rear immersed tube section is completed, a steel seal door is dismounted, the axis reinforcement and connectors and the vertical reinforcement and connectors on the front immersed tube sleeper beam section and the rear immersed tube sleeper beam section are adjusted, and axis reinforcements on the front immersed tube sleeper beam section and axis reinforcements on the rear immersed tube sleeper beam section are shorn in a crossing manner; and meanwhile, the vertical reinforcements and the axis reinforcements cross, the crossed axis reinforcements and vertical reinforcements are bound, and then concrete pouring is carried out. According to the method for manufacturing the horizontal concrete shearing key of the tunnel immersed tube joint, the sleeper beam axis reinforcement and connectors and the vertical reinforcement and connectors are buried when the sleeper beams are prefabricated, the vertical reinforcements meeting a shearing key structure are connected through the reinforcement after the sleeper beams are slightly finished and chiseled, finally, the aim of utilizing an existing concrete structure is achieved, and the economic and environmental protection effects are achieved.