In the arc additive manufacturing of aluminum alloy, different frequencies (30 KHz, 40 KHz, 50 KHz) of ultrasonic stirring were applied in the molten pool using a stirring needle moving synchronously with the arc, and the results showed that 1) the organization of the additive layer showed grain refinement, a decrease in the number of columnar crystals, and an increase in the number of equiaxed crystals compared with that without ultrasonic stirring; and the grain refinement became more obvious as the ultrasonic frequency increased; 2) the porosity of the additive layer ultrasonic stirring tended to decrease with increasing ultrasonic frequency, 0.73%, 0.64%, and 0.59%, respectively, but increased compared to the specimen without ultrasonic stirring (0.2%); 3) the tensile strength of the accretion layer was 266.57 MPa (without ultrasonic), 278 MPa, 282 MPa, and 299 MPa, respectively, and the elongation was 30.67% (without ultrasonic) 31.54%, 35.53%, 41.86%; with the increase of ultrasonic stirring frequency, the strength and plasticity of the reinforced layer were increased.