The invention discloses a surface treatment device and method for material-increase manufacturing plastic products. The device comprises a treatment kettle, a solvent, a workpiece basket, a heater, a temperature sensor and a driving unit for driving products to be treated or the solvent to move; the workpiece basket is arranged in the treatment kettle; the products to be treated are placed in the workpiece basket; the solvent is placed in the treatment kettle; the heater is used for heating the solvent; when the solvent temperature detected by the temperature sensor reaches a preset temperature, the driving unit is started, so that the solvent and the products to be treated generate relative movement. According to the surface treatment device of material-increase manufacturing plastic products, when the solvent temperature detected by the temperature sensor reaches the preset temperature, the driving unit is started, so that the solvent and the products to be treated generate relative movement, therefore, bulges on the surfaces of the products to be treated are easier to be dissolved, less recesses of the products to be treated are dissolved. Therefore, the purposes of effectively reducing the surface roughness and improving the quality of the surfaces of the products are achieved.