With a transformer plus rectifier bridge to do a simple 12V100A output DC power supply, the switch is opened on the 21V spike voltage, how to reduce this to 18V?

May be radiation interference, should be caused by the oscilloscope probe, there may be around the SCR tuning equipment, or did not come with power correction using 220V to DC equipment.

Directly short-circuit the probe, adjust the attenuation, synchronization, to see if there is a spike

Possibly *** mode interference,

Generally in the filter capacitor will not have a spike. Headphones can be strung a capacitor to see if there is no AC sound, pay attention to the connection, do not let the capacitor shock current damage headphones.

Just saw that you are nearly 1000W load, is the core saturation caused by spikes? You can reduce the load to try,

You can add inductance to try, but 100A load, inductance is not good

You can and TVS to try, but the voltage may not be so accurate,

Our aging room, the original use of a simple rectifier power supply, and then in order to improve the quality of the switching power supply with power factor correction.

Based on your follow-up question, it does not seem like interference, you can look at the transformer output waveform, may no longer be sinusoidal, the input to do a current transformer, look at the input current waveform, may no longer be a simple pulse waveform.

Find the cause and then solve it

For reference only