The invention relates to a device for stabilizing a mechanical arm in an in-and-out movement process of an electrolytic manganese cathode plate from a tank. The device comprises a guide rail, walking wheels, a support beam, an in-and-out tank part and a movement part, wherein at least two walking wheels are placed on the guide rail and are capable of sliding on the guide rail; the support beam is connected to the walking wheel and suspended on the guide rail; the movement part is suspended under the support beam through two movement lifting cylinders on two sides; the in-and-out tank part is also suspended under the support bam through two in-and-out tank lifting cylinders on the two sides; the movement part is located above the in-and-out tank part. Through the device for stabilizing the mechanical arm in the in-and-out movement process of the electrolytic manganese cathode plate from the tank, the mechanical treatment of a process for taking the cathode plate in and out of the tank in a subsequent section is realized; the conventional manual driving is not used taking the cathode plate in and out of the tank; the situation of uneven stress and easiness in deflection of the cathode plate can be avoided; the balance of the cathode plate can be maintained well without extra auxiliary personnel, so that the personnel cost is reduced, and the working efficiency is improved.