The invention discloses a steam drive oil well downhole injection agent injection method and device. The device comprises a pipe column assembly arranged in a casing pipe. An oil jacket annulus is arranged between the pipe column assembly and the casing pipe. The pipe column assembly comprises a sieve pipe, an injection agent pipe column and a minitype screw pump pipe column which are sequentially connected from top to bottom. The upper end of the sieve pipe is connected with an oil pumping pipe column. The injection agent pipe column is filled with an injection agent. A U-shaped baffle bowl is arranged in an inner cavity, above the injection agent liquid level, in the injection agent pipe column. The minitype screw pump pipe column is internally provided with a minitype screw pump. A flow baffle device is arranged on the outer side of the lower end of the minitype screw pump pipe column. A downward V-shaped packing cup is installed on the flow baffle device to baffle well liquid from flowing towards the oil jacket annulus. The minitype screw pump pipe column is provided with a radial well liquid flowing-out way and an injection agent flowing-out way. The steam drive oil well downhole injection agent injection method and device depend on well liquid flow driving, and a special drive device is not needed; no floor space is occupied, and the hydrogen sulfide removing process is completely finished underground.