Selective laser melting (SLM) technology provides an effective solution for the fabrication of microchannel heat exchanger. However, the polishing post-treatment of the inner hole surface of the microchannel with complex orientation has always been a technical problem which restricts its wide application. Abrasive flow polishing technology was applied to polish the inner surface of SLM additive manufacturing microchannel. The influence of abrasive flow polishing on the surface quality of microchannel and the shape accuracy of inner hole under different polishing pressure was studied. The flow trajectory of abrasive was simulated by FLUENT software to explore the mechanism of abrasive flow polishing on the wall of microchannel. In the same polishing time, the surface roughness of the microchannel decreases and tends to converge to a minimum value with the increase of polishing pressure. The roughness of the straight pipe can reach 0.23 μm, and the outside of the elbow is 0.24 μm. The polishing effect is obvious. At the same time, the maximum inscribed circle diameter and minimum circumscribed circle diameter of inner hole of microchannel increase, and the change trend of straight pipe is relatively stable. When the polishing pressure is less than 5 MPa, the change trend of the maximum inscribed circle diameter and the minimum circumscribed circle diameter is relatively stable. When the polishing pressure is greater than 5 MPa, the change trend of the maximum inscribed circle diameter and the minimum circumscribed circle diameter is large, and the shape of the inner hole of the microchannel changes greatly. When the polishing time is 20 min and the polishing pressure is less than 5 MPa, not only has microchannel good surface quality, but also its inner hole shape will not be deformed greatly, and the shape accuracy is better. Under this processing condition, it can meet the processing requirements.