The invention relates to a long-tail-pipe well washing protector for preventing corrosion of a casing. A cavity is formed in an upper shunting joint, a valve ball is placed in the cavity, a valve seat is mounted in the upper shunting joint, the upper part of a shunting pipe and the lower part of the upper shunting joint are connected by internal threads, the upper end face of the shunting pipe props against the lower end face of the valve seat, a screwed plug is connected with the upper shunting joint, the upper shunting joint, a packer and a lower shunting joint are sequentially connected, the lower part of the shunting pipe is inserted into an inner hole of the lower shunting joint, a sealing ring is mounted between the shunting pipe and the lower shunting joint, a lateral outlet is formed in the upper shunting joint, a vertical flow path is formed in the upper shunting joint, an annular channel is formed between the shunting pipe and the packer, a lateral inlet is formed in the lower shunting joint, and a vertical flow path is formed in the lower shunting joint. The long-tail-pipe well washing protector for preventing corrosion of the casing can be matched and used with a long-tail-pipe oil pumping pipe column for preventing corrosion of the casing, can prevent corrosion of the casing, and can prevent an oil layer from being polluted in the well washing process.