Gear shifting and speed changing method, gearbox and engineering machinery

The invention relates to the technical field of gearboxes, and provides a gear shifting and speed changing method, a gearbox and engineering machinery. The gearbox comprises an input shaft, an output shaft and a plurality of clutches, and the plurality of clutches comprise a plurality of primary clutches and a plurality of secondary clutches. The gear shifting and speed changing method comprises the steps that when a gear I is shifted to a gear M, the clutch which is not engaged with the gear M runs for an idle stroke; when the clutch which is not engaged with the gear M finishes the idle stroke, the clutch, different from the clutch corresponding to the gear M, in the gear I is disengaged; and after the clutch, different from the clutch corresponding to the gear M, in the gear I is disengaged, the clutch which is not engaged with the gear M is engaged. Therefore, along with the disengagement of the second-stage clutch of the gear I, the power of the input shaft quickly starts to be transmitted to the first-s