Abstract: To achieve integrated optimization of the macro-micro forming quality of 300M steel during plasma arc additive manufacturing, the welding current, powder feeding rate and scanning speed were used as process parameters to design an orthogonal experiment. The results show that the influence degree of process parameters on the macroscopic forming quality and microscopic defects of the deposited layer is not the same. Therefore, a multi-dimensional evaluation system encompassing macroscopic morphology, geometric characteristics and microscopic defects was established to systematically optimize the process parameters for plasma arc additive manufacturing of 300M steel by multi-objective optimization approach. The results show that the influence degree of process parameters on the macro-micro forming quality follows the order: scanning speed＞welding current＞powder feeding rate. The optimal combination of process parameters obtained through optimization is as follows: welding current of 160 A, scanning speed of 0.14 m/min and powder feeding rate of 15 g/min. The forming quality of the deposited layer under the optimized process parameters achieves the optimal balance between the multidimensional evaluation indexes. The surface of the deposited layer is smooth and flat, unstable weld-up areas and jagged boundaries are eliminated, the aspect ratio is 4.29, the porosity is 0.43%, and there are no large pore defects.