The invention discloses a field implementing method for online change of a catalyst of a purifying pre-shift converter. The field implementing method for online change of the catalyst of the purifyingpre-shift converter comprises the following steps: S1, modifying the pre-shift converter before online switch-out, adding a first gate valve at the front end of a first blind plate at an inlet of a shift converter, and adding second gate valves at the front ends of the two positions, with second blind plates, of a technically modified pipeline; S2, when a system runs normally, opening the first blind plate, enabling the first gate valve to be in a totally closed state, opening the second blind plates, a third blind plate, a fourth blind plate and a fifth blind plate, and enabling the second gate valve to be opened totally; S3, preparing to transfer the pre-shift converter for repairing by the system; S4, preparing to use the pre-shift converter by the system. By control over switches of two gate valve doors including the first newly added gate valve and the second newly added gate valve, unplugging and plugging of the blind plates of the system and adjustment of the process, the pre-shift converter can be used or stopped, and the field implementing method is simple to operate and high in safety; and most of pre-shift catalysts in the invention are catalysts with poor activity obtained by change of the converter subsequently, under the condition that normal production is maintained, the cost is reduced, and the production benefit is increased.