The invention discloses an assembling method of a large rotary drum type flying shear non-clearance synchronizing mechanism. The assembling method comprises the following steps of 1), assembling a transmission side gear pair, enabling an inner hole of a synchronous gear III is in interference fit with an upper rotary drum shaft neck in assembling, and enabling an inner hole of a synchronous gear IV is in interference fit with a lower rotary drum shaft neck in assembling; 2), assembling an operation side gear pair, enabling an inner hole of a synchronous gear II is in interference fit with theupper rotary drum shaft neck in assembling, and enabling an inner hole of a synchronous gear I is in interference fit with the lower rotary drum shaft neck in assembling; 3), adjusting the side gap ofthe transmission side gear pair, enabling the inner hole of the synchronous gear IV is in interference fit with the lower rotary drum shaft neck in assembling, and enabling the inner hole of the synchronous gear V is in interference fit with the lower rotary drum shaft neck in assembling; 4), adjusting the clearance-free synchronous mechanism; 5), judging the side clearance direction of the clearance-free synchronous mechanism; and 6), adjusting the contact surface of the gear pair on the operation side. According to the method, the adjusting precision is high, the manufacturing cost is reduced, the time is saved, and the assembly efficiency is improved.