Can the negative pole of the DC switching power supply be directly grounded?

Under normal circumstances, there is no problem, but in fact, your 12V is derived from 220V, but the 12V power supply is independent of the 220V power supply. The 220V ground wire is commonly known as the earth, in order to play the role of shielding and leakage protection. If the two ground wires are connected together, when the 220V leaks, 220V electricity will pass through the ground wire. Because the electric potential difference of 12V is superimposed on 220V, the 12V which is originally a safe voltage may cause electric shock, resulting in accidents occur.

Are you directly connected for electromagnetic compatibility, choose resistance-capacitance grounding, and generally choose floating ground for DC grounding. In different power supply systems, if leakage occurs, the shell is grounded, and the ground level is no longer zero potential. Whether the low-voltage distribution system is TN-C or TN-S protective ground is different when the leakage occurs.