The invention relates to a control method and system for auxiliary gear engaging of a wet-type double-clutch synchronizer and an automobile. The method specifically comprises the steps that S1, rotational inertia of all gears is obtained, whether the rotational inertia of a target gear is minimum or not is judged, if yes, gear engaging is conducted towards the target gear, and the process is ended, and if not, the step S2 is executed; s2, enabling an auxiliary gear shifting fork to enter a gear engaging state; s3, the auxiliary gear is made to enter a gear taking-off state, and an auxiliary gear shifting fork is made to be located in a neutral gear area; and S4, the target gear is made to enter the gear engaging state till the target gear shifting fork moves to the in-gear position till gear engaging of the target gear shifting fork is completed. According to the method, when the low gear is engaged in a static state, the idle high gear can be used for engaging the gear, the rotating speed of the target gear clutch is reduced, and then the low gear is engaged, so that the gear engaging is completed by using small gear engaging force, the gear engaging noise is reduced, and the comfort of the whole vehicle is improved.